First of all, from an environmental point of view, my personal opinion is that nobody needs a house larger than around 4,000 square feet. Anything bigger is not for need, but ego.
In a way, I could be considered a hypocrite. I am a craftsman and specialty contractor for a product mostly used on upscale homes. I currently have 3 homes in progress that range in size from 10,000 to 16,000 square feet. The price tags for these homes are $7mil, $9mil and $14 mil. I don't advertise and just end up working on whatever my word-of-mouth reputation sends my way.
I do have some insight as to what goes on in the mega-home market and the people that build them. For $6mil, you could build a very classy, elegant home with 6,000 to 9,000 square feet and virtually every amenity anyone could wish for. Or you could build a 28,000 square foot box of crap. Oh sure, with careful budgeting, there would be enough to put nice nice finishes on your box of crap. But making that choice reveals a lot about the homeowner. While the first choice could be made by someone I would consider presidential, the second screams trailer-park lotto winner.