It's a common misconception that there are few overweight east Asians: http://www.hindawi.com/journals/jobes/2010/868573/tab1/

Further, much of the population of east Asia lives in poverty or near-poverty conditions. Their portion sizes are small (i.e., total daily carbohydrate intake is nowhere even remotely close to that of developed nations). In fact, half a billion Chinese are living on less than $2 / day: http://en.wikipedia.org/wiki/Poverty_in_the_Peoples_Republic...

Also, east Asians eat whole grain brown rice, not the fried, bleached, processed, white rice that Americans typically eat, which is largely devoid of nutrients and fiber.

For far more detail, visit page 226 of Gary Taubes' 'Why we get fat' book.