Japan has a "work hard" culture and a high percentage of employment, not to mention many extracurricular activities in school that encourage physical activity. They generally promote the well being of their citizens, if only to further the well-being of the country.
I don't know enough about south korea except to say they are obsessed with self-image.
Genetics might also play a role but i'm thinking it has more to do with diet. Lack of cheap beef, traditional low-fat foods, lean meats like fish or chicken and less deepfrying in general.