Over there high calorie food is all around you all the time, and everyone’s eating tons of it all the time; it’s completely normal behaviour. It’s like (I imagine) being an alcoholic and the whole world being a pub.
Sitting in Starbucks overhearing the orders, almost everyone is ordering a venti super sweet high fat drink, and it’s people in their cars on their way to work so it looks like a daily thing.
No idea how anyone can eat the quantities they do
One point: just because the portion is huge, doesn’t mean they eat it. It’s one of those cultural things; a huge portion is seen as better value, better hospitality, or largesse and you aren’t actually intended to finish it. Most people I know expect a meal out to last two meals or more, you take the rest home in a box.