I don’t think there would be so many fast food restaurants if food were not so cheap. Or, if there were, they would be different from the fast food restaurants we have now.
Food being cheap also gives restaurants an incentive to serve larger portions. They can get a reputation for giving people a lot of food for not very much money, which most people think is a desirable thing, at not much cost to them. That’s a win-win for the restaurant. But it means people get used to eating more food at each meal. If you start eating more at every meal, with no change in what kind of food you’re eating, of course you’re going to gain weight.
What’s really a problem, IMO, is when you have food attitudes from a time when food wasn’t cheap and abundant, in a setting where food actually is cheap and abundant. “Clean your plate” isn’t the right response when you’re eating in a restaurant that serves a 2000 calorie plate of food (as casual chain restaurants in the US do), especially not if you’re eating at that kind of restaurant several times a week (as many Americans do). If you’re overweight, seeking out ways to get a lot of food for little money may not be what you should be doing, since you’d be better off eating less food. But if attitudes like “clean your plate, even if you’re feeling full,” “don’t waste food,” and “a lot of food for little money is a good thing” since childhood, you’re going to have trouble throwing off those ideas, even when those ideas are not helping you live in the world as it is now.
I know this, from personal experience. I’ve been trying for a couple of years to train myself out of cleaning my plate. It hasn’t been easy. I was raised by a mother who was a child in the 1940s, right at the spike in that graph of food expenditures as a percentage of income. She would have picked up attitudes toward food that would be suitable to an environment where food is not cheap and plentiful. Then she taught those attitudes to her children, who live in an environment that’s pretty much the opposite. My sister and I both struggle with being overweight, not too surprisingly. It’s as if we were raised in Siberia, where we presumably would have been taught lots of ways to stay warm, then moved to Florida, where it is generally not difficult to stay warm. Behaviors that would be suitable in Siberia might lead to heatstroke or other problems in Florida’s very different environment.
I’m not blaming my mother, or anybody else who grew up during World War II, here. They were doing what they thought was best, and adapting to a new environment is almost always difficult. But I’m not planning to try to pass the same attitudes toward food that I learned from my parents on to my child. We live in a different environment now.