In my opinion, it takes GREAT effort to get fat in Japan. The culture, lifestyle, and overall diet essentially makes you shed pounds without you even realizing it.

In order to be overweight, you would seriously have to down buckets of ice cream, eat over dozens of bigmacs everyday, and chug bottles of cooking oil for breakfast, lunch, and dinner. Am I exaggerating? Only a bit. Do some people get fat anyway? Sure, but it's only a small minority.

If you want to find out a Japanese person's reaction to being overweight, just think of it like this: If I told you that I just ran a 20 mile marathon and gained 10 pounds, what would your reaction be? Probably be somewhere along the lines of "WTF how did that happen? There must be something seriously wrong with you." If you're a foreign visitor, I'm sure they'll be more polite about it. But if you're really obese, then expect a lot of WTF reactions and maybe even some teasing from the older folks.