It's a strange time for houses to get bigger. The housing bust was followed by sharp criticisms of excesses such as McMansions and started a movement toward tiny houses. Combined with America's aging, "empty nester" population, increasing environmental concerns and smaller household sizes, you might think the U.S. would want to build smaller houses. We haven't.

As of the last Census, the median house was 2,384 square feet—in between the sizes of a single and a doubles tennis courts. That's up from 1,525 square feet in...