Talk to car salespeople. Large american vehicles have nothing to do with law and everything to do with the american consumer preferring larger vehicles. You can see it in this very thread, they want to "feel" safer.
It's probably a mixture of both, but don't underestimate the impact of laws that make large cars/trucks cheaper and that make smaller cars cost prohibitive.
I hauled two kids around in a Toyota Matrix (Corolla station wagon) for years, and never had a problem with the car seat not fitting or making me feel like a sardine.
