Being obese as a kid is almost causal for being obese later in life[1] as becoming obese screws up a lot of your bodies biology permenantly. You can of course change and become healthier but many lingering symptoms linger regardless of you losing weight. While still 70% obese adults were not obese as children 80% of obese children end up being obese.
Open to other ideas but school meals and peoples relationship with food is extremely important to maintaining weight in my experience.
There might also be a genetic factor, why japanese are less obese or overweight, because the difference for diabetes patients between US and japan is a lot smaller.