I dont see why FC's need mansions. I know why they WANT them, and its the same reason I want them.I see no point in 1 person owning a Mansion sized house over a FC with a large amount of people.
I have a personal Large, 2 alt FC's with Larges, and my main FC is in a medium. I spend more time in any of my houses in a day than anyone but me has spent in my active FC's house total the last year, and that was the person who decorated it with me. The only people I ever see in my personal ward are 3 people who have apartments there and come out to use the marketboard right next to me.
Housing is one of those things that is, for the overwhelming majority of people, ONLY interesting to the homeowner/decorator.