My theory is two-pronged.
First, for thousands of years, most of the people of the world have lived hand-to-mouth. Sitting about idle one day meant not being able to eat at least one other day. Bodily fat reserves were genuinely useful to smooth the cycles of feast and famine, especially for women during childbearing years.
Second, there was a very real famine in the Western World, especially America 70 years ago. My parents grew up during that time, called the Great Depression, when many couldn’t find much more than the very minimal necessities of life including good nutrition.
Then, war came and the people starved even more. When Europe finally crawled out of those horrible years, it nevertheless had a good millenium of culinary practice and tradition to look back on. Americans looked back to the semi-penury of the frontier; their culinary traditions, outside of just a few intact ethnic groups like the Italians, were largely non-existant.
What quickly set into America, in a subtle way, was a struggle to offer restaurants to people whose ancestors had never been able to afford going out to eat and, at the same time, the unconscious pressure to eat everything one could lay one’s hands on pushed the expected quantities up. In short, once Americans could afford to eat, they didn’t stop until their ever expanding stomachs said enough. They had never really eaten their fill before, they demanded huge quantities of food from smorgasbord style restaurants to justify spending money for food outside the home.
While today the smorgasbord with its crowds of senior citizens almost ritually stuffing themselves has been marginalized somewhat (though these places can still be found all over the United States), dining out at your local Mexican restaurant is enough to remind you that restaurant practice in our country is still based on a value-quantity formula.
Some of my family, mostly co-laterals, were the size of refrigerators. I have written computer software since I was 25 and was a draftsman prior to that. I weigh nearly twice what I weighed in high school.