As I understand it, the European Smarts are much lighter and thus more fuel efficient; the US government (perhaps in a move of trade protectionism) required a lot of extra body reinforcement for Smart cars before they would allow their import to the US. At least the last time I checked, their fuel economy was on par with other four-seaters (hybrids, 3-cyls) I looked at.
I find it far more likely that the extra strength was required for the impact collisions. The other cars on the road in the US are just far larger and massive than those elsewhere, so a stronger heavier car is needed to stay safe.
It creates a bit of a Catch 22 when trying to bring down the average size / weight of a vehicle.
