I don't agree that this is what's going on.
People don't avoid obesity by exceptional willpower. They just have healthy eating habits, usually ones formed during childhood.
Likewise, overcoming obesity and returning oneself to a healthier weight that one can maintain requires changing habits and one's relationship with food.
Habit formation and modification can be difficult, but it's sufficiently well understood that it can be implemented.
The difficulty is getting information into people's hands about how to modify habits successfully (I like the book The Power of Habit), cutting through the misinformation surrounding what constitutes healthy eating habits and how to lose weight, and often just education about how to cook food cheaply and in a way that doesn't seem intimidating to people who are pressed for time and resort to fast food out of convenience.
When people know the concrete steps one must take to achieve something, they become that much more willing to believe they can do it.
Advertising is no doubt a factor in the obesity epidemic, but I'm not sure that's among the primary causes of it. It seems to me that food portion sizes have increased tremendously over the past four decades. Purveyors of food often seem to peddle the sheer size of portions as features. Additionally, foods that once might have been thought outrageously decadent, such as "coffee" covered in whipped-cream and loaded with caramel (basically a dessert in a cup), and pizza with crust infused with cheese, are now perceived as normal things to consume, quite possibly on a daily basis. My non-scientific observation is that the normalization of consumption of huge portions of calorie-dense "foods" is directly correlated with increases in obesity. Since we're not willing to de-normalize the former as far as I can tell, the emerging societal trend is to normalize the latter.
[0] "http://www.livescience.com/49-decade-study-americans-taller-...
Are people in low-obesity countries more resistant to such biological impulses? Do they have fewer food ads? Are they able to resist those advertisers in a more coherent fashion? Or just exhibit better willpower at national level?
This question touches on some very deep and controversial ideas kicking around in the current conversation. The question of whether we have an inherent "human nature", a biological substrate that informs our capabilities and behavior, touches on everything from education to tech employment. I don't want to debate the subject generally.
But can we agree, for the specific case of food consumption that
1) we have very strong impulses that are difficult to consciously resist; that
2) these impulses originate in biology; that
3) the existence of exceptional willpower is a mark of exceptional self-discipline, a level not practical to achieve for most; and that
4) advertisers prey on the difficulty most people experience in controlling their food behavior?