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 are 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.
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. And the power armor is automated anyway, so they move for him at the speed of thought.)
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 Khadoran army. This is most pronounced with Vlad Tzepesci, 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 supposedly to protect the head from a warjack punch, but really it's just because PP's concept artists have pauldron envy. When Warmachine: Tactics came out, a computer game set in the Warmachine world, the pauldrons actually had to be downsized so the animation team could render things like swinging an arm upward without decapitation.
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 a Tau Fashion
'cough' Armor envy 'cough'
Summarised In a Necrony Fashion
-.. --- -. .----. - ..-. --- .-. --. . - - --- -.. .-. .. -. -.- -.-- --- ..- .-. --- ...- .- .-.. - .. -. .
-. . -.-. .-. --- -. ... -.. --- -. .----. - -. . . -.. .--. .- ..- .-.. -.. .-. --- -. ... --..-- .- .-.. ... --- --..-- -.-- --- ..- .... .- ...- . .-- .- -.-- - --- --- -- ..- -.-. .... - .. -- . --- -. -.-- --- ..- .-. .... .- -. -.. ... .. -- . .- -. ... . .-. .. --- ..- ... .-.. -.-- --..-- .-- .... -.-- .-- --- ..- .-.. -.. -.-- --- ..- -... --- - .... . .-. - .-. .- -. ... .-.. .- - .. -. --. .- .-.. .-.. --- ..-. - .... .. ... .-.-.- .- .-.. ... --- -.. --- -. .----. - ..-. --- .-. --. . - - .... .- - --- ...- .- .-.. - .. -. . .- -. -.. .. .... --- .--. . -.-- --- ..- --. . - - .... .- - .-. . ..-. . .-. . -. -.-. .
Summarised In an Orky Fashion
'ERE BOSS, YOO WAN GET US SUM OF DEM METEL PLATES? ILL GET DA MEK.
Summarised for Tyranids
Pauldrons are hard to get through, but when you do... pauldron wearers taste good. KEKEKEKEKEKEKEKEZERGRUSHDEVOURNOMNOMNOMNOM
Summarised by the Imperial Guard
Armor is for PUSSIES!
Summarised in Dark Eldar fashion
Heh, you know what they say about hunks with huge pauldrons... *fapfapfap*
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 shit out.
also: they help hide cracks and joints between the arm and the body thus allowing you to cover up modelling mistakes, or allow for a bit more flexibility in posing.