Aggression is a wholly ineffective behavioural change effector. You are just being a short-sighted ass, and the fact that your simplistic opinion is shared by most of the citizenry is most likely the largest cause of obesity.
You won't understand why until you consider that the biggest cause of obesity is psychological.
Many people have a hard time understanding what psychological issues are and how real they manifest themselves. It's not unlike the middle ages where ignorant healers would bleed you to try and get rid of the sickness. These are opinions based on whatever common sense they had at the time combined with a general ignorance. These people were not dumb, they were just uninformed. Now you straighten yourself out.
People get fat because their psychological state drives them to consume things that produce dopamine (the hormone that makes you happy). Probably because they either don't have enough of it (they're sad) or because they've grown addicted to it (nearly everything you buy nowadays will make you addicted to dopamine). To solve the "getting fat" problem, people need to stay away from unhealthy things that produce dopamine (sadly, these are also the "easy" things), and start finding the healthy things that produce dopamine (going out with friends, learning, experiencing new things). Sadly, this becomes harder and harder as your weight increases.
But that's not all. Once you're heavy, solving the "getting fat" problem not only gets tougher, it also won't actually make you skinny. Even if you stop eating anything unhealthy, you will not lose weight. You could eat half the calories a healthy skinny person eats and not lose weight. That's because your body is designed to not go down in weight. You can do crazy things to go down temporarily, but your body will be fighting you all the way and as soon as it gets the chance it will reset your weight back to what it was. This is why nearly every dieter regains their weight. To lose weight permanently, you need to either fight your body's set-point permanently or undergo a certain type of surgery, such as a gastroscopic bypass or duodenal switch.
As for why your attitude is what causes obesity: simplification of the issue, making it taboo and agressively pushing skinnyness are all factors which cause both the psychological environment where a person will start to obsess over the importance of their weight, as well as the bad sources of dopamine and the physical situation of people starving themselves for no good reason which will have the result of your body going into panic mode, shut down its metabolism and build stores of fat for anything it can possibly get its hands on.
The best way to make your population fat is to tell them being fat is horrible, all your own fault and eating food is bad for you. For the love of all that is good, DO NOT TELL ANY CHILD TO NOT GET FAT. Just teach them to live happy and healthy. Being happy means you need no bad sources of dopamine.