I've spent time in Japan too (I majored in Japanese and I lived there for a year), and, while Japanese people don't have very much tact about body image issues, I find self-reports like that very unconvincing. Japanese people will also wax poetic about the uniqueness of their having four seasons, which would probably be news to most of my fellow New Englanders.

There are a lot of other things different about Japan (most people walk more and drive rather infrequently, food portions are smaller, the actual foods eaten are not the same, for instance) than just how tactless or not Japanese people are about weight. I bet if you looked at Nikkei with Japanese immigrant parents you would see rates of obesity that looked much more similar to American ones than Japanese ones too.

In addition, although it's a slower creep, Japan is seeing average BMIs go up.