The census data linked does break it down by region (Northeast, Midwest, South, and West) and by city versus rural (Inside MSA and Outside MSA). It also lists both average and median. The exact rate of increase obviously varies depending on these facts, but the pattern is the same, American houses today are just bigger.

OK,

Actually looking at your data, it's the size of newly built houses. So definitely the houses themselves are getting bigger.

But with the occupants of all the houses, old and new, getting on average poorer, I'd be most interested in the average and median square footage that Americans live in and I don't think your data really gives data on that.

I would also add in the usable space (number of usable rooms). Many houses come with a formal dining room combined with a formal living room (often called the great room); they then have a family room that flows into the kitchen. Add in an oversized master bedroom, and you have quite a bit of square footage but not really anything you can do with it that you couldn't do with smaller houses.