I have been to over 16 countries, where most of our ancestors have come from, and I will tell you.... it's not genetic!
Americans are overweight because (in addition to what jenniferhughs said), our farmers pump the meat we eat and the milk we drink with growth hormones and steriods. This is illegal in most of our sister nations (i.e. the UK, Australia, and New Zealand). When writing your article, look at a study done comparing Asian women to American-asian women. Asians don't drink cow's milk, they drink soy. While American-Asian women drink cow's milk. The difference is, the American women are developing large breasts at young ages (10-14), while the Asian girls look normal for their ages.