It's not like it is impossible to be thin on a high carb diet. Some things that come to my mind are:
- Hara Hachi Bu - you now, the "eat until 80% full"-thing: so far I've only heard that this is part of Okinawan culture but maybe people do in general show a little bit more restraint when it comes to eating? It is not only about what kind of crap you eat but also about portion sizes. And portion sizes are definitely not big there from my experience. To me it always seems as if the Japanese tend to be a little more "in control" about various aspects of life, maybe that is one of them.
- A japanese friend of mine once told me that no, I was mistaken in thinking that asian women just automatically tend to stay thin because of their genetic makeup (that was my theory and I'm still not convinced otherwise! ). She told me that when they gain a little weight, many women just stop eating because there is even more pressure from society to be thin. IIRC, her quote was "We don't really have boobs so we need to have long, slim legs to make up for it. Asian people are all about legs". Based on the TV commercials that you usually see on Japanese TV, I definitely believe her that there is much more external pressure from the media to be thin, white and perfect in every way.
- "They do walk on average about an hour a day": well, that's at least something and probably more than the average American walks per day but probably not the most important factor