What monstro said, above.
A lot of people have no idea what a proper portion size is. That’s step one. Step 2 is to figure out what you’re overeating on and try to find a substitute. Like crunch and salt? Carrots and seasoned salt. Sweets? Fruit. It works. As my doctor told me (when he asked how I’d lost weight without going on about it or having any “counseling” or “treatment”, “No one ever gained weight by eating cherries.”)
Step 3 is to take some exercise. It won’t make you lose weight on its own, but it sure doesn’t hurt.
Anyway, I think that our more sedentary lives, **plus **easy availability of junk food, **plus **the normalization of huge portions, all add up to people moving from “plump” into “needs mobility scooter”/“corpse being weighed on the large animal vet school scale”/“needs extra-large hospital bed” territory.
I also suspect that Americans’ use of drugs might play into this. As far as I know, studies are done on particular medications and side effects reported for those meds. And maybe studies are done on two medications being taken at the same time. But it’s normal for reasonably healthy people to take multiple drugs, all the time, and we don’t know how those might be affecting our appetites or metabolisms long-term.