i think you are right.
i have always seen big portion sizes in restaurants when i have gone to the US.
also, very recently, i saw a youtube video in which a french person says the same thing. they said that even though the French eat a lot of fat, their overall portion sizes are smaller than those of the US. so overall, they end up eating less calories than US people.
also the French tend to walk a lot, while the US people tend to drive a lot.
it's a generalization, i know.