I think what you mean to say is that larges have the most space, which nobody disagrees with. Item to space ratio refers to number of blocks divided by of items which is absolutely abysmal for large houses. Whether you block off space doesn't change the fact that it's there. Maybe you personally would only decorate the foyer if you had 2000 item slots but many people block off the Foyer because decorating the entire mansion with 400 items and make it look well-designed is extremely difficult due to the poor item to space ratio.