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. Or so the Mechanicus would have you believe - in truth, pauldrons simply redirect all incoming projectiles back to the enemy so the wearer doesn't even have to move. In the absence of incoming projectiles, they instead draw in and focus all available sunlight on the target, which is why the setting has no light and is so Grimdark.
With regards to space marines in particular, the the big round upper-arm-covering pauldrons are so ubiquitous that using a different design (see the Primarchs or Cypher's new model) has become easy visual shorthand for 'this character is a big fucking deal'.
Those unfortunate enough not to be equipped with power armor instead use the size of their hats in a similar manner. (although most guard regiments, particularly cadians, tend to wear pauldrons rather than more useful armour like gauntlets or legpads)
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. This is most prominent in the Khadoran army, but recent releases for the Retribution of Scyrah have challenged Khadoran pauldron supremacy. A prime example is 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.
Our enemy wears MEHTAL BAWKSES, DA COWHARDS, DA FUHLES! we... weeee shall wear bigger meatle bawkses...
We prefer pauldron-sized helmets. Maugan Ra loves those pauldrons though...
cough' Armor envy 'cough' Actually, proportionately ours are bigger.
-.. --- -. .----. - ..-. --- .-. --. . - - --- -.. .-. .. -. -.- -.-- --- ..- .-. --- ...- .- .-.. - .. -. .
-. . -.-. .-. --- -. ... -.. --- -. .----. - -. . . -.. .--. .- ..- .-.. -.. .-. --- -. ... --..-- .- .-.. ... --- --..-- -.-- --- ..- .... .- ...- . .-- .- -.-- - --- --- -- ..- -.-. .... - .. -- . --- -. -.-- --- ..- .-. .... .- -. -.. ... .. -- . .- -. ... . .-. .. --- ..- ... .-.. -.-- --..-- .-- .... -.-- .-- --- ..- .-.. -.. -.-- --- ..- -... --- - .... . .-. - .-. .- -. ... .-.. .- - .. -. --. .- .-.. .-.. --- ..-. - .... .. ... .-.-.- .- .-.. ... --- -.. --- -. .----. - ..-. --- .-. --. . - - .... .- - --- ...- .- .-.. - .. -. . .- -. -.. .. .... --- .--. . -.-- --- ..- --. . - - .... .- - .-. . ..-. . .-. . -. -.-. .
( DON'T FORGET TO DRINK YOUR OVALTINE NECRONS DON'T NEED PAULDRONS, ALSO, YOU HAVE WAY TOO MUCH TIME ON YOUR HANDS I MEAN SERIOUSLY, WHY WOULD YOU BOTHER TRANSLATING ALL OF THIS. ALSO DON'T FORGET THAT OVALTINE AND I HOPE YOU GET THAT REFERENCE.)
'ERE BOSS, YOO WAN GET US SUM OF DEM METEL PLATES? ILL GET DA MEK.
Pauldrons are hard to get through, but when you do... pauldron wearers taste good. KEKEKEKEKEKEKEKEZERGRUSHDEVOURNOMNOMNOMNOM
Summarised by the Imperial Guard
Armor is for PU-*BLAM*
Summarised in Dark Eldar fashion
Heh, you know what they say about hunks with huge pauldrons... *fapfapfap*
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. In the case of Space Marines they're also vaguely evocative of a Papal mantle, assisting their Catholic space-monk imagery, although whether this is intentional or just a happy accident is up for debate. And, also, bigger pauldrons do make it easier to freehand sigils for custom chapters and junk.