Why do you expect a rational person would necessarily avoid the environmental problems that cause overweight and obesity? Especially given that scientists are very unclear amongst themselves as to what causes obesity and weight gain? Even if you adhere to the notion that weight gain and loss is simply a matter of calorie consumption and willpower, why would you assume a rational person has more willpower?

A more rational person might have a better understanding of how his mind works and use that understanding to deploy his limited willpower to maximum effect.