Mediums and larges have upper floors, but smalls only have a basement. Several in game items show an upper floor in the exterior anyway. Increase the space for items, and give us one more floor to decorate. It's a weird design choice when the two larger sizes than you have upper floors, and we don't. I get the size differences, but they should all have an equal number of floors, just smaller.

It would also make sense considering the fact that houses are hard to come by, and that smalls are the most common. It would help bring more housing equality to all.