I think there's a lot to it...think about the Scandinavians in Norway and Sweden....not a lot of hugely obese people there....then think about them in the US....not to pick on Scandinavians. Germans are similar. There are very few really fat Europeans (I lived there for 11 years so I have some exposure). Having said that, obesity can still be a disease...no one says it has to be purely hereditary.
Having read the Omnivore's Dilemma, I do think that the culprit is the US food industry which, in line with market forces, has created thousands of ways to indulge your tastebuds, almost entirely with sugar-based stimulation. If you think back to what was available foodwise 50 years ago, you just weren't likely to want to eat enough to get truly obese.
I mean a McDonalds meal is designed to suck you in with the salty fries and the sugary burger and then make you hungry again in an hour or so.