Everything is proportionally smaller in Japan, including fast food. American portion sizes are seen as ridiculously oversized. There's also mandatory yearly checkups paid for by companies and if you are overweight, they will scold you and you can get in trouble at work for it, which is bad when you plan to stay at the same company your entire life, so it tends to keep people healthy and thin.
Japanese people are far more perturbed by perceived American loudness, brashness, ignorance and lack of intelligence than their eating habits.
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There are ways to indulge though. Buffets are pretty rare, but yakiniku is a great way to get a lot of calories, and some Japanese people eat a frankly unhealthy amount of rice. My personal favourite fat food is this platter from GoGo Curry which costs under $10. Plus you can always just buy two servings if you want.
Then every now and again they will just throw out some massive food item as a gimmick, like the burger above, or the massive McDonalds fries they do sometimes. Or the Giga big mac. You can also double the meat on any Mcdonalds Burger for $1 after 5pm, so a Double cheeseburger has 4 patties etc.
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It's not hard for a fast food loving fatty like me though to chow down regularly.