I've been in 7,000-12,000 sqft homes. My cousin had "friends" who lived in such houses. These homes were in an enclave that was part of a master-planned community that included townhomes, and single-family homes starting around 2,500 sq.ft.
Those massive buildings are not homes, they are cold, empty, and have a museum feel. You don't want to touch anything, you're in awe of expensive art and sculpture and furnishings.
I don't know how people would sleep in there... I'd be seeing ghosts. And I haven't mentioned the utility bills; they interior is around 73F in the peak of the Florida Summer. I hope they installed solar panels.
Maybe downsizing comes naturally to people who don't need much in the house.