Are rich people on average fatter than thin people? After all, they can afford more food, so presumably they consume more of it.

Spending has dropped because there are such large subsidies to cheap carbs, and that is the bulk of many people's diets. A steak and veggies is no cheaper than it was 40-50 years ago, but we're eating far more refined junk with additives because it is cheap and convenient.

I'm not quite sure what your first point is? Infamously, rich poor are on average thinner (the "poverty obesity" paradox).

I believe the leading hypothesis is that being rich absolves you of the stressors that make junk food attractive. Others are, as you mentioned, the ability to buy more expensive food with more nourishment/satiation per calorie ("protein leverage hypothesis"), or access to nature and refuge from pollution (it's certainly the rich people where I live who ride bicycles.).

Regardless, a poor person from 50 years ago couldn't buy the cheap sugary crap even if they wanted to.

My post was in response to the parent's statements "If I read the USDA charts, it’s about 13% for the decade of the 70s, declining to under 10% today." and "So I would expect that as food gets significantly cheaper, people consume more of it."

That is, the point of the parent's post was not obviously "crap food is so much cheaper than high quality food", but as most naturally read as "because we can afford more food, we're buying and eating more"

My point was "if that logic holds, I would expect the rich to be significantly fatter than the poor". Which, as you note, is not the case. Nothing more.

Portions are now massive. McDonalds, once upon a time, had small and large sizes of french fries; the little white paper pouch or the red cardboard sleeve. Today I believe they have two even larger sizes. The smallest size of soda doesn’t even exist any more.

Other restaurants have retired round plates and serve meals on platters.

I also think cheap food tends to be worse for you on average.