Let them go as big as they want, but they're heavily taxed on any residence they control (it can't be "owned" because there are ways around that word) but don't live in for at least six months plus one day out of the year. Make this true for everyone. We're not telling anyone they can't have a second "vacation" house, just that it won't be cheap and certainly not profitable. Doesn't apply to multifamily rental properties of four or more units. Single family home rentals are destroyed, suddenly the market is flooded with cheap homes.

Also, we need to make it international, in that they'll be taxed at home for any residences controlled overseas. Of course, they'll hide them behind shell businesses owned through Nevis, but that's a different problem that should also be addressed.