A lot of it has to do with diet. Most Asian-Americans still eat a lot of homecooked Asian food*, as opposed to burgers, fries and pizza like most Americans. Basically a lot of meat & cheese. * Which is also nowhere near as greasy as the stuff they serve in Asian restaurants.
If it were genetics, then you'd see a lot of fat White people in the EU, Latinos in Latin America and Black people in Africa.
Surprise, Surprise, though. White Euros are very slim...and I believe Black Africans and many Latin Americans are too. Heck, White Americans HERE a generation ago were all fairly in shape.
The biggest common denominator here then is MODERN AMERICAN FOOD...and you'll notice that Asian people who have assimilated to the diet & lifestyle here also blow up. Although that is not as common because most never fully assimilate to the food here.
Sure, genetics might play some part, but I think diet can override that pretty quickly.