We're a huge country I guess. I've experienced pretty much the exact opposite as you.

2. almost everyone I know takes home some portion of their meal from a restaurant. So that single portion is really two, or maybe three.

Most people around me have the waiter take their half-eaten plate away. Apparently, food waste is considered polite or something around me. If you actually try to finish your plate, you get made fun of. (I've been made fun of.) I guess it makes you look poor or desperate or starving or something?

3. 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.

The dinner plates my family and my wife's family (and extended family) have for dinner are quite large and they usually get filled up. Usually meat is the biggest portion of that plate, followed by carbs, and then veggies are the smallest (if present at all). My wife's family in particular always, always, always, has ice cream or cake or cookies after dinner.

My wife and I use smaller dinner plates, and again, surprise, get made fun of because we're eating such "tiny" (normal) portions.