It’s not that simple: Americans aren’t unusually fat because everyone set out to get that way, but because we have unhealthy lifestyles (sedentary driving is the primary travel mode, we live in car-centric exercise-discouraging housing, our jobs tend to be sedentary but high-stress / long hours, etc.) and heavily subsidize low-quality food, and we price healthcare so much higher than the rest of the world that many people are discouraged from preventative treatment. If you fix any combination of those, obesity rates will go down but you’d have to do some care analysis to say how much of the healthcare expenditure reductions were due to lifestyle improvements rather than weight loss.