Weight loss is simply calories in vs calories out. Americans have far larger proportions than most other countries. Japan is very reliant on public transportation so a lot of people walk to work and not drive meaning they at least burn more energy than the average American. And like you said their diet of seafood, rice, and plants is healthier and has less calories. Americans have heavily subsidized their beef, wheat, and corn, so getting a giant burger, or a massive soda is super cheap because all the ingredients are cheap. Lot of factors at play.

I think that's the seafood and rice diet, not smaller soda cups doing that.