There is, I think, a few things that contribute here.
The US has a very stupid “bigger is better” mentality. So if you go out you expect a large portion because that translates to better (and more value). This is of course not true, but culturally it’s very embedded.
almost everyone I know takes home some portion of their meal from a restaurant. So that single portion is really two, or maybe three.
IME people don’t usually have giant portions at home, they sometimes do of course, but things tend to be more sane for home cooked meals for your family. They also tend to be a lot more balanced, with more veg and grain.
what you see on TV is often sensationalized, and not fully indicative of normal here.
There is, I think, a few things that contribute here.