I kinda look at these McMansions today as kind of like what a Cadillac or Lincoln became in the 70's. Prior to that, a Caddy or Lincoln was an expensive, exlusive automobile. You didn't see them everywhere you turned, and you had to have money to buy one...at least a new one.
By the 70's though, even though the times were rough, somehow they must have been easier to obtain, either with more people having more income or longer-term car loans, or both, because suddenly sales of both brands exploded. With the exception of a few bad years like 1974-75, when fuel was expensive/scarce.
But suddenly everybody had these things, and they weren't so "special" anymore. And to this day they really aren't. A big, roomy house used to be much more special when it was comparatively rare. I remember as a kid, the three biggest houses in my zip code were probably about 3,000 square feet each, if that. And older houses, built before 1900. Back then they really stood out for their size, and today they have a style that makes them unique. But they're simply dwarfed by these McMansions that are shooting up all over the place. Now we don't have any of the 10,000 square foot monsters sprouting up around here, but there seem to be plenty in the 3500-5000 square foot range. And even if the square footage isn't so massive, they give them large, imposing roofs to make them look bigger than they really are.
Back in 1999, one of my co-workers had this ~5000 square foot monster built out in Annapolis. It was on a 2 acre lot that had a lot of woods, so it wasn't too out-of-place. But it was just so generic looking, and you could tell where they cut corners. For example, the whole thing was just sided in generic, white vinyl. No brick, stone, stucco veneer or anything to break up the monotony. And the front porch just had lattice skirting around it, like what you'd expect to see in a trailer park! Now I'll admit, my grandma's house has lattice skirting under her front porch, and used to have some under the back deck until the dogs tore it apart, but on her modest little cape cod, it just didn't seem so out of place.