I have consistently lost weight on both trips I took to Japan despite not necessarily eating very healthily. I think a large part of it is all the stairs/passageways into and out of train stations. Obviously some have escalators but some don't or during rush hour it's easier to just take the steps. In the Bay Area we don't have a transportation system with that kind of infrastructure. New York City's subways would be a bit similar, but most of the stations I've been to there have much fewer stairways.
Anecdotal, but this mirrors my experience as well; without fail I lose weight on every trip. I’m not necessarily doing that much touristy stuff either but I walk significantly more than I do back home (I also work from home which probably doesn’t help). Part of it might be that I have more time as I’m in Japan for a holiday but I also find most of Japan more interesting to walk around and the cities are more densely packed with fewer stretches of empty land. I also don’t have a car which probably forces more walking but I often actively chose not to catch a train as well, even if it makes the trip a little longer.
