Even apart from path dependence, increasing the volume of a room by 25% must increase the materials cost and heating cost by some amount, if only 10%. More surface to paint, more difficulty reaching light bulbs, taller ladders to your gutters, and so on. Not free.

I find this kind of logic sufficiently compelling that I have never rented more than a single room, so I guess I was never going to agree with OP in the first place.