Houses began growing in the 50s as the postwar baby boom and growing prosperity both incentivized larger homes, and it really took off through the 60s. There was a sort of pause during the 70s recession, but it all picked up again with the McMansion trend of the 80s and 90s.

It's also intended as a starter home, with a nursery or kids room. Seniors may want a guest room. And it could also be an office space or sewing/craft room. A healthy neighborhood should have houses for all sages of life intermingled, in my opinion, and homes for young families aren't that different from retirement homes.

I'm pondering a more open plan, but that trend has pros and cons alike. One of my versions I am trying is more open for the kitchen, dining, and living spaces.