It's not just willpower but also lifestyle. It's rare for people who are physically active, and have a balanced diet, to suffer from obesity. I can eat A LOT without putting on a ton of weight, and it's because the types of foods I eat and because I do strength training, which means have a fair amount of muscle mass which acts as metabolic furnace. I'm a little overweight, at the moment, but it'd take a lot of effort on my part to become obese. I think treating obesity as a health condition is the wrong approach.