I'd say it's a legitimate design decision that fits in with, first and foremost, iron sights, as well as health regeneration, reduced accuracy when moving/increased accuracy when crouching, limited sprint ability, silent movement while walking, one-hit-kill headshots, &c. &c. It's all part of making the player slow down, rather than run-and-gun through everything. (In this respect, I don't think Deus Ex would have suffered from a larger view of the weapon/iron sights.)
If you were making a run-and-gun style thing, and a gun did take up this much space (without it being e.g. a specific disadvantage for using that weapon), or if using iron sights didn't improve visibility (I'm looking at you, Killing Floor's M14), that would be a problem.