I got a little bit of a chuckle out of this thread, because the original assumption behind it was shown to be inaccurate ... but that didn't stop anyone from continuing to explain it!
If you look back at cars from the 60s and some earlier decades, the Crosstrek is actually a pretty lightweight vehicle ... the Chrysler I drove back in college was close to 4,000 pounds, for example. The energy crisis of the 70s was a big factor in starting to reduce car weights, but that trend was offset in recent years by America's desire for ever-larger vehicles. Those giant modern three-row SUVs and massive pickup trucks are the things that should be called morbidly obese -- compared to most of what's on the road today, the Crosstrek is pretty svelte, which is one of the reasons I bought it. And considering its safety and capability, I'd say it's a pretty remarkably efficient use of weight.