Why are so many Americans fat?

why are there so many overweight Americans? No, I am not talking about woman who naturally have some curves or no flat belly - that's perfectly normal, I mean BIG people. I go to cities and I see these people who act entirely unaccountable for their state of morbid obesity. It's sad in a way, and hard to watch what people do to themselves. The "rich people's disease" of obesity seems uniquely American - but why?