During The Great Recession, the small-is-better crowd seemed to be winning. After decades of upsizing and the spread of suburban McMansions, the average size of new U.S. single-family homes fell. Yes, it actually shrank.

Architects such as Sarah Susanka, author of the Not So Big House series, cheered the slight downsizing—about 5 percent from 2007 to 2010. They wondered whether their less-is-more view had finally become the new Zeitgeist or whether Americans were simply strapped for cash.

Turns out, it was just the economy. The shrinking didn’t last, and new U.S. homes are now bigger than ever, according to the most recent Census Bureau data. But how bad is this for the environment, since larger homes typically use more energy?

There’s some good news today on that front. Efficiency gains are offsetting more than 70 percent of the growth in energy use that would result from the increasing size and number of U.S. households, reports the U.S. Energy Information Administration.

In fact, energy intensity—energy used per square foot—was 37 percent lower (or better) in 2009 than in 1980. It meant a reduced use of coal, natural gas and nuclear fuel.

Why this progress? The EIA cites factors that include energy prices, shifts in fuel sources and stricter building codes. It also notes the broader use of more efficient technologies—from appliances and lighting to heating/cooling units, some of which were promoted via energy labeling programs such as the voluntary Energy Star.

That’s the good news. The EIA also gives the bad: “The gains from energy intensity improvements would have been even larger if it were not for consumer preferences for larger homes and increased adoption of home appliances and electronics.”

In the three-decade period studied, the average home size grew about 20%. With more square footage came more and larger devices, such as big-screen TVs that gulp energy. So U.S. households actually used more energy overall, 10.2 quadrillion British thermal units (quads) in 2009—up from 9.3 quads in 1980—even though they used less per square foot.

Moral of the story: Efficiency matters but so does size. An accomplished architect I know was once asked by a client how to make a new 10,000 square-foot home “green.” His response: Don’t build it.