Obesity is at once simple and complex. It's simple, because each of use chooses what to eat and how much to exercise. But it's obviously so much more than that. I think it starts with our food and farm policies, and goes from there all the way to the way kids are raised to be scared to leave the house anymore. And probably a lot in between.
It's funny. My brother and I were raised the same, ate the same things, played the same sports (hockey, tennis, swimming, golf--can you guess what race we are?!), and have the same education. Being only a year apart, we had an almost identical upbringing. But in our adulthoods we've completely diverged. He eats junk food and doesn't exercise, and I pretty much eat healthy(ish), while exercising almost every day. We're both in our thirties, and our physiques, as you can imagine, are quite a bit different by now. Which is odd, because he was always a way better athlete than me growing up. In fact, maybe that's why I am the way I am; I always felt like I had to keep up (I'm about 4" shorter than him, 6'0" vs 5'8", a big difference, and something I've always been self conscious of), so I never gave up. Whereas in his case, he always was the leader and everything came naturally to him.
My point is that even all things being equal, the psychology of the individual plays into it a lot. It's not all government failure and corporate greed (although I would certainly argue that that's the biggest component). A great place to start to combat obesity is to flip the subsidies that farmers receive. Subsidize the fuck out of vegetables, subsidize fruits a tiny bit less, and do everything humanly possible to disincentivize people from eating super calorie dense garbage.