As a frenchman, I agree portions in the US are quite big, especially in lower/middle class chains where quantity >quality. Ridiculous tier: all you can eat buffets.
I also agree there are sometimes more processed ingredients and weird additives (calcium in OJ anyone?) in store bought foods and beverage, but that's just because different régulations. Like in France you can't legally call a juice "juice" if there is sugar or water added.
This is also a matter of taste...I think what people are sometimes butthurt at is the way americans treat the products they like, or that are originating from their countries. For instance, mayonnaise: as a frenchman I grew up on home made, raw unpasterized eggs mayo, and it obviously bears little ressemblance to Heinz mayo. I do feel Heinz mayo works best on fries tho, and I'm gratefeul for the way americans gave me a chance to enjoy different variations of a same recipe.
But there are plenty of alternatives, and you are never forced to go eat at a Denny's or feed off gas station hot dogs, nachos and mega gulp's. Do some fatasfatasses do it, 3 times a day? Yes. Is it widely available, and to such a scale that it's very cheap? Yes? Is it the only definition of US food? Nope.
You can (and should) have some basics in nutrition, and work out a meal from a restaurant menu. You can just order an entree, or share a main with another person from your party. I see many americans doing it.
Most restaurants I've been to in the US were extremely keen on removing stuf from a dish, or serving on the side, and it seems it is more about you being socially awkward than the US food and hospitality industry to be to blame.
BTW I do enjoy this difference with Europe, I like fine dining as well as hearty popular foods, and while I'm on holidays in the US I actually love eating in chains and fast foods that we don't have home, as well as fancier places. I have never been yet, but am looking forward to enjoying some Southern food next time.