The Japanese seem to go in extremes when it comes to weight -- they're either very slender or they're a Sumo wrestler. It seems like you rarely see a 'mildly chubby' person from Japan. So what's their secret (for the skinny, not the sumo)? According to this article it's probably these 5 things:

• In Japan more people walk or ride their bikes to get around, and few drive. Natural, built-in exercise.
• The Japanese eat more fish than any other culture. Sushi, of course, but also many other types of cooked fish and seafood.
• Tofu is a staple of most Japanese diets and it's been shown to reduce risk of heart disease and cancer.
  
• People eat slower in Japan. Partially it's the culture, and partially it's the chopsticks, but it serves the purpose of giving the brain time to realize the stomach is full before they overeat.
• People in Japan generally eat more fresh vegetables, and less canned and frozen ones. They also eat more vegetables overall than Americans (and other cultures).

Via Fitsugar