Because the typical Chinese household doesn't eat too much food. It's all about portion control and they eat a ton of veggies. Rice is just a side dish to them, and they only eat it once or twice a day.
Also, you need to take into account that the typical Chinese peasant family is out farming or working hard all day. They need a lot of carbs to work that hard. Now that they have a significant population living in large cities and eating "American" fast food, working in office jobs, they are beginning to have a lot of the same health problems (obesity) that we do.
Also, you need to take into account that the typical Chinese peasant family is out farming or working hard all day. They need a lot of carbs to work that hard. Now that they have a significant population living in large cities and eating "American" fast food, working in office jobs, they are beginning to have a lot of the same health problems (obesity) that we do.