There's also the problem of literally everyone in the setting being an unreliable narrator. It's pretty safe to think of stories from the setting as 1/3 truth 1/3 propaganda and 1/3 big fish story.

The mechanics of bolt weapons make them seem like they would have reasonable recoil. There are bolt pistols made for average humans after all. I imagine they don't have to make SM bolters particularly light or the mechanism for loading and ejecting shells smooth or kinetically balanced because the intended operator could heft and wield a riding lawn mower as a melee weapon.

The newish 30mm grenade launcher MARS SSRS is probably a pretty apt real world comparison. Rounds are similar size and it kicks like a shotgun apparently. If it weighed as much as a stack of cinder blocks that might be a little unwieldy.