Fantasy Armor

Oh boy...

The short version of it is that some people involved in game designing (both traditional and video games) tend to design female armor to be more sexually attractive than functional. This topic can lead to lively debates.

Categorization

Here's a brief rundown...

Category Zero: A female character is simply clad in regular armor that is identical to what male characters have and is purely functional in design. If it has any ornamentation, it is simply the sort of ornamentation that is common for that region. Some examples include chainmail shirt and a helmet, a suit of plate, gear worn by modern female soldiers in combat situations, etc.
Category One: Armor is specifically made to be worn by women, but is still functional. Ornamentation, if present can be more feminine. Think of the difference between a man's and a woman's suit. Cat One armor is either as good or better for a female wearer than Cat Zero armor.
Category Two: (Boobplate) Specific to plate armor, this involves a pair of boobs being hammered into a breastplate. This will direct a sword blow (or worse: a bullet) inward towards the heart. A simple raised section in the armor to accommodate breasts would fall into Category One. Special note has to go to the real world Greek Muscle cuirass which had sculpted boob plates, only it was worn by men (ie the Sanguinary Guard), and further only by officers, making it a rare case of male category 2 or 1. That said, since a six pack and abs is flatter than a pair of boobs it's much less of an issue.
Category Three: Female armor which is really really tightly fitting, thus making it hard to put on or take off, decreasing its effectiveness as armor and making it hard to move in.
Category Four: Armor with cleavage, mid-riffs and similar, leaving portions of the body exposed for blades, spears and arrows. Also appearing at this point are high heels, and not the kind intended for riding.
Category Five: Chainmail and plate bikinis. At this point we are not dealing with armor but rather armor-themed stripper costumes whose only purpose is for pure fanservice. The male example is roman gladiator armor which usually only consisted of armor on a single arm and a belt, and, like Cat 5 armor, it was intended mostly for spectacle rather than utility.
Category Six: Included only for completeness sake, a cat 6 is no armor and no clothing, runing into battle completely naked. This was historically done by males of certain cultures such as the Norse and Celts. Interestingly the Celts and Vikings were also known for having female warriors meaning category 1 and category 3 and 6 (more so Celts than Vikings), as none of these people used plate armor, this is the most likely option on this list for West-specific historical accuracy and "artistic license of historical events", which is kinda pathetic if you think about it. Scythians and Sarmatians would fall under category under category one.

Ubiquity

Even for all it's silliness, the bigger issue is not that this stuff exists. If it was something which cropped up once in a blue moon it would be dismissed as a quirk of a few series who are a bit more fanservice heavy than others. The real issue that it has become effectively the norm rather than the exception. That said, there has been a bit of a backlash against this in recent years.

Perspectives on Female Fantasy Armor

On the one hand you have people who are against revealing female armor. It's self evidently badly designed for the purpose of protecting it's wearer and exists for reasons of cheap fanservice. That it has been allowed to become the norm also reflects poorly on people who like the genre, making them come off as juvenile and sexist.

Then you have it's proponents/defenders. Said people would argue that it adds flavor to the setting and that it looks nice. When confronted with it's deficiency in terms of protection, some people will try to defend them by making points such as "women being unable to carry as much weight as men". In general terms this is true, but mostly this has to do with upper body strength in the arms, strength in the legs and spine is more even between individuals of both sexes of comparable size and build and armor is mostly carried by the shoulders for the same reason a backpack is, though females are typically of a lighter build than males. Which means that the actual problem with this argument is that it ignores that a smaller person has less area for armor to cover, and thus the weight of the armor is lighter. Another oft quoted and lacking of understanding of physics is "increased mobility", leaving aside the matter of high heels or the cumbersomeness of wearing an armored corset. The only named fictional character who gets away with this is the current Samus, because her high heels are actually jet-heels [1] which also allows her to kick opponents in the face and burn them at the same time. Characters who come from horse riding cultures also have excusable heels (to a degree, stilettos are still stupid) and can also get away with high heels since the "heel" was originally designed to work with a stirrups and fashion just extended it to a stupid degree. The 'socially pragmatic' card is "distraction" assuming that all men are dumb and horny enough to automatically let their guards down to stare at tits, double sexism ho! Others, who realize that it's a bad design would bring up that A) The name of the genre is Fantasy and its defined by moving trees, animate statues and giant armored winged reptiles that can fly and spew fire out of their mouths without incinerating their tongue, so unrealistic elements are to be expected and B) in real life women seldom served as warriors during the middle ages in any case.

A third group exists which settles somewhere in between the previously mentioned groups. These are people who feel that it depends on the setting. If it's a setting where male and female soldiers fight in pike formations and phalanxes supported by companies of crossbow armed soldiers using volley fire, halberdiers, mounted archers and lancers and otherwise aims for a degree of realism, have reasonable and functionally designed female armor. For more overtly fantastical stuff and stuff like Conan the Barbarian where you have guys wearing a single pauldron, a bandoleer for a sword, a kilt and a pair of boots into battle, the chainmail bikinis fit the tone and are fine. Notably, Dark Sun was specifically designed as a desert world solely to make this style of armor make sense and because the developers forgot that actual desert dwellers have to wear concealing full-body outfits to prevent sunstroke.

Female Armor in Real Life

In the ancient times, there were many instances of women on the battlefield. The Sarmatians, who gave the Persians, Greeks and Romans hell in the form of horse archers outflanking and outrunning heavier or more numerous units if any of those people set foot in their territory. They used bronze armor, purportedly, though given their closeness and possible part of Scythian civilization it is more likely they used what the Scythians used, chainmail, who fought employing all sexes as well. An ancient Queen of Scythia Tomryas, was the one to behead King Cyrus of Persia on the field of battle, marking a clear boundary for the Persian Empire's Eastern borders, and also being the number one reason most discussions of and books or shows about Cyrus by classical West-centric and Middle East-centric thinkers do not usually cover how he died, since they avoid mention of independent women who win against empires as much as possible and would prefer to sexualize Cleopatra as a femme fatale and depict Boudicca wearing a chain-mail bikini or leathery equivalent before they were invented (the chainmail/leather bikini being late 20th century and the first proper example Red Sonja. It should be noted that Conan beat her scantily clad barbarian routine by several decades though) if they even bother to mention Boudicca. Many Celtic tribes men and women both went to war, in berserker with an iron longsword style, which usually meant, and depending on the tribe, that most people ran into battle wearing only pants and with blue warpaint all over their bodies, and some were more tactical (or lucky to have the resources) to employ bronze and iron armors. The only exceptions are Druids and the wealthier/royal members of a tribe. To give them credit, they did usually use a testudo-like or heavy shield of some sort and chariots. We even have records of Roman female Gladiators, called gladiatrices. Thought Japenses history, but especially in the Waring states period, a women trained in the use of the Naginata was consider an especially valuable wife since she had the skill to defend against attackers while the husband was off fighting. In fact in the empire of Japan for school children Karate and Kendo were compulsory for boys, while Naginata-do, the art of using a Naginata, was mandatory for school girls.

However, you will notice that the civilizations known for female warriors were not highly agrarian, at least not compared to Rome, Medevial Europe or Japan (though Japan is a bit odd in this regard as seen as above but they were not expected to fight offensively though it did happen.). In farming premodern warfare women did not serve on the front lines as soldiers as a general rule and so In the middle ages (before armor was fazed out anyway in favor of cheap musketeers) and as such generally did not wear armor. The reason for this was that armor was expensive, took a lot of time to make and generally they wore what armor was immediately available on hand. There were exceptions that came up now and again (Joan of Arc in France, Onna Bushi in Japan) and when it happened they wore armor basically identical to male armor, at most falling into Category One. In modern times, many first world armies allow women to serve as soldiers, in which case they wear basically the same combat uniforms as their male comrades in arms do and are about as flattering. Though this is somewhat of a problem: a soldier's gear is highly standardized (including their armor)and set to a number of size variations, and unfortunately for female soldiers, "woman" is not one of them. This often also includes soldiers of an uncommon body shape (aka huge dudes), and even people who are left handed given how are rifles eject spent shells, making it a problem across the entire armed forces. As uniforms along with almost everything else first world militaries use is made by private industries, this is either them cutting costs while still making a profit that most other industries can only dream of, being chauvinists or just lazy.

And if you think that this is the reason that women should either stop bitching or not join the military, please go teabag a bear trap.

Gallery

Jenette Vasquez from Aliens in Category Zero. As seen with Hudson as a comparison, her armor looks exactly like the ones male marines use (apart from the extra equipment used to allow her to use a Smartgun system).
Elspeth Tirel shows off a suit of Category One: while being without a helmet and with a flowing long robe this is the standard suit of armor for the knights of Bant, who will not exploit these two weaknesses when fighting their honorable duels.
Sister Miriya of the Sisters of Battle in Category Two. Category Two armor is generally the most acceptable form of female armor for most fantasy fans since it logically looks like it can protect the user while remaining aesthetically pleasing.
An Eldar Farseer in Category three. Although, while it is tight-fitting, Eldar mesh armor is designed to be tight while still allowing users free movement. Plus, while the picture is of a woman, Eldar men wear the exact same armor making this an out of universe type three, but in universe type two since she still has a boob plate (to clarify, male Eldar banshees are expected by Khaine to wear these too, and the other 12 Warrior Paths and Guardians completely lack it, but Autarchs and Farseers use it.)
Lelith Hesperax in a Category Four/Five Hybrid. While it can't protect her, it is nearly impossible to strike Lelith in the first place due to her borderline-supernatural agility, and so her armor is optimized to be as lightweight(and thus easy to maneuver in) as possible. Additionally, Dark Eldar culture demands that Wyches and Succubi be as sexually appealing as they are deadly in combat- a Wych that isn't attractive enough for the crowds inevitably ends up in an arena match that she can't win and the male members of wych cults dress about as skimpily.
A male category 2 muscle cuirass. It has largely the same problems as female category two. They even went to the trouble of hammering out nipples and a navel.
A male category 4/5 historical roman gladiator. The exposed chest is considered to be the gladiators badge of honor. If you've pierced that, you've pierced the gladiators honour. No. His other honour.