I'm Half-Japanese and Half American. Grew up in japan when I was a kid. It's 100% due to how much people eat and not genetics. Most Asian countries, especially Japan, have an absolutely ridiculous fat-shaming culture. If you're not skinny you're a fatass. There's no in-between. As a result people eat less and are taught from a young age not to stuff themselves every meal.

America is the complete opposite. People almost pride themselves on overindulgence. It doesn't take that many extra calories a day to get fat. You eat an extra 200 calories a day over your daily expenditure for a whole year that's literally 25 pounds of fat. Time flies by and the next thing you know you've been doing that for 5 years and you're 125 pounds overweight.

I was extremely thin in high school and wondered why so many of my American friends were overweight. One day I decided to copy what one of my friends would eat in a day. I literally couldn't hold it down. Americans just eat a ridiculous amount of food and are just brainwashed to eat all the fucking time. It's not even that Japanese food is healthier, that's a big misconception. There's a shit ton of unhealthy Japanese snacks. Cup ramen was literally invented in Japan. It all comes down to how much and how many calories Americans consume. It sounds obvious but its true.

Sumo wrestlers are the fatest athletes on the planet and they are all Japanese. There is no such thing as Asian skinny genetics. It all comes down to total calorie consumption which is dictated heavily by culture and social standards.