The body weight is a question of personal choises and culture, let's say if a person lives in a comunity where people who eat to much are rejected then the habits wouldn't be the same.
Obesity is not a personal choice it's a medical condition dude ...
I believe that bad food habits / bad food education are the biggest problem.
After that I'd say, bad quality of products/GMO used over there.
And finaly, I'd say the success of advertisement on weakminded/poorly educated people is to blame too
.