Ezra Klein's "Research Desk" pulls together some data on home sizes in the United States and abroad. American houses are larger than their counterparts in Europe and, as you can see from the chart above, were getting larger and larger until just recently.
The big house trend is bad for the planet in a variety of ways. Large homes are, of course, more costly from an energy perspective. But if the demand for large houses is satisfied, in part, by developing in far-flung suburbs where land is cheap, then that also encourages lots of unnecessary transportation. Perhaps the drop in average home size in the last few years reflects a change in values—the idea that luxury is measured by efficiency rather than size seems to be gaining currency—but it's hard to know for sure. It might just be the collapse of the housing market.