Pauldrons
In the grim darkness of the far future/Renaissance Europe, there is only war. Unless you have Imperial Power Armor/Chaos Plate or Norscan Marauder's hide armor, in which case there is also pauldrons. Pauldrons are the sections of armor that cover the shoulders, and are well known for the tendency of artists drawing Warhammer 40k material to make them ridiculously huge. World of Warcraft armor also tends towards oversized shoulder-plates. (Fuck you, Blizzard!)
In the far far future... Manliness is measured in the size of your pauldrons.
It is theorized that position in Imperial society is directly related to how large your pauldrons are, as exemplified by artwork of the Empra himself, who has power armor with pauldrons so massive he cannot see over his own shoulders or raise his arms to more than a forty-five degree angle.(Imperial Power armor bypasses this inhibition by having their pauldrons being automated. Thus when an Astartes must look over his shoulders, the pauldrons will automatically lower. As for moving their limbs fluidly, one must keep in mind that the average Space Marines has tremendous physical strength, and thus would be capable of lifting his arms fluidly despite the weight of the armor.)
Those unfortunate enough not to be equipped with power armor instead use the size of their hats in a similar manner.
Privateer Press has copied the style of the huge pauldrons of Warhammer 40k and added comically large pauldrons to some of the characters in its Warmachine game, mostly in the Khadorian army. This is most pronounced with Vlad Tzipesci, aka "Lord No-Peripheral-Vision", to the extent that the pads on Vlad's third incarnation are probably a massive troll. A few other models, like the Great Bears of Gallowswood, are certainly not innocent either. The fluff reason is to protect the head for a warjack punch, as a normal set of armor would result in a snapped neck, meaning a decent reason.
Summarised In a Chaosy Fashion
Our enemy wears METAL BAWKSES, DA COWARDS, DA FUHLES! we... weeee shall wear bigger metal bawkses...
Summarised In a Eldary Fashion
What are those Mon'keigh doing?
Summarised In an Orky Fashion
'ERE BOSS, YOO WAN GET US SUM OF DEM SHINEY PLATES?
Summarised for Tyranids
Pauldron wearers taste good. KEKEKEKEKEKEKEKEZERGRUSHDEVOURNOMNOMNOMNOM
The Truth
The real, secret reason that Imperial society features such massive pauldrons is so that Games Workshop can claim that their armour is distinctive enough to justify suing the pants off any rivals/imitators/wellmeaning fans who produce anything that could be used with their products. Seriously, check this link out: http://www.courthousenews.com/2012/12/04/52810.htm
also: they help hide cracks and joints between the arm and the body thus allow you to put the arm slightly off center to get some extra flexibility in pose without this being visible.
