Very interesting question and even more so with the responses. I too, never knew the true definition of Mcmansion and always perceived them to be large communities of homes built to appear and contain almost identical characteristic on the outside or as Papi Chulo quoted, "lacking in architectural integrity"
The only unknown is what amount of square footage would justify a home a Mcmansion and fell within the other defined factors above. I always perceived these sorts of homes or communities were strictly on the coasts however that is incorrect since i'm in the middle of the country now, moving from west to east and I see the same features in all the communities around here. there might not be as many homes in a particular community but the builders greed for money is easily identified whether from the distances between the homes, the cheap fixtures used in new homes, or the evidence of shoddy workmanship when touring the model homes in new communities.
I followed the rules, well most of them, in my previous life on west coast. The young starting out family bought a condo early phase of brand new sprawling master planned community, quickly sold after 2 years, and bought the bigger brand new single family home to accommodate our increasing family size. The builder requested all the buyers complete a survey once they closed and moved into home, my only remark on the survey was i believed the home would not outlive the life of the loan. I didn't know what to really look for when we were buying when it came to craftsmanship or quality although, i was pretty sure i wasn't supposed to feel half sunken nail heads under my feet when i walked on our upgraded carpet and padding during the initial days of living in the home.
Now, i lease a brand new home in the middle of the country and to me, it's even worse with the craftsmanship and quality of fixtures and the owner wanted offered to sell it to me for 450k and i so badly wanted to tell him i wouldn't touch it for 350K. And for reference, none of these homes were over 3000sqft either 4-5brms but nothing insanely huge in my eyes.
someone said there waiting for the valley or downturn, i'm doing the same. I've kept an adequate amount of my proceeds so when this thing bottoms and it will, I will take my pick of what will serve my needs and wants. My only two issues/concerns will any of us still be here when it bottoms and do i want to go further off grid or back to civilization and the debauchery i sorely miss.
TL/DR:
Practically all, new homes under 7 figs are built very poorly, have minimal variance to neighbors home, and the lot sizes are no more than 1/3 acre with most being much smaller those our homes where i came from on west side of country and that's being generous.