GWriter saidAmericans do tend to be chunky. But paradoxically, we also work out more. Our "average" is probably a bimodal distribution if this is taken into account. I've travelled internationally quite a bit, and whenever I notice a guy who is muscular or in especially good shape, he is almost always another American tourist (occasionally Australian). With a few exceptions, most of the world just isn't that into lifting weights. Try finding a real gym abroad!
I think that a small percentage of Americans are super fit/muscular. However, over 30% are outright obese, and if you consider how many people are borderline obese or overweight to an unhealthy degree, that brings the percentage to about 50-60. Of the remaining population, most are in a sort of "normal" range. But I don't think that "fitness" is merely confined to being muscle bound or lifting weights. To me, it's more about general lifestyle. Take the example of the average Dutch guy. The average Dutch person rides his bike to work and to do everyday commuting. You rarely see a truly obese Dutch person. He might not be at the gym getting bulky, but overall, he's more active.