Outdated cargo ship. I dunno, maybe the air recycler's inefficient, so they have to have a lot of air contained in there already. Maybe the bulkheads are so huge, nad in order to withstand high Gs, need to be so straight and inflexible that this room becomes excessively large. Maybe the equipment in the room (nav, etc. mainframes continuing, from their visible giant consiles, under the floor, equipment for super-high-res screens on wall, who knows what else) is so massive that there is a certain bubble of space they surround anyway. Real reason, it looks cool.