Why are Americans so obese?

Just something I've noticed after traveling around Asia, Canada and E. Europe. My best guess is Americans, on average, carry about 20lbs more fat than other nationalities. Even the Mexican-Americans seem to be obese. Whenever I come to the US I gain 20lbs and lose it as soon as I leave. I've noticed everything tastes sweeter and/or saltier in the US and has MSG(or similar flavor enhancers) added. An hour after eating a big meal at Taco Bell I feel hungry again.