I think this is mostly a social/societal thing - at an early age in schools they tell kids that they should only eat until they're 80% full. And there's substantial social pressure and bullying of anyone considered even mildly overweight.

Also, most people have a lot of walking/biking built into their daily schedule, especially in larger cities where having a car is impractical.

This all means that while there is a huge amount of sweets and fatty food, it's usually eaten in moderation, and people get exercise in their daily lives to work it off.