Actually, I think it's because in general, people in Asian countries are far more active physically than those of us in the US. Think about it...do you walk or ride a bike nearly everywhere you go? Do you work long hours at physically demanding jobs (I'm thinking of Asians in rural areas here)? If you're a person who is moving constantly and expending a lot of physical energy every day, you can get away with eating more carbs than someone who isn't.
Also, don't think that because they are slim, Asians are more healthy. Stroke rates are far higher in Asian countries than many others and new cases of diabetes is rising faster in Asian countries right now than any other country in the world. Obesity among children is also on the rise even in countries like Japan and China.
Something else that many people don't realize is that rice is about the only high carb food that many Asians eat; the rest of their diets are generally very low in carbs and high in vegetables. They also love pork, lard and coconut oil and use them liberally in their diets.