This is a recent video and has me questioning what I believe. He shows us our history with food and the possible reason for obesity is environmental factors that lead to metabolism disruption.
I think the video is based off limited information. Some things to consider:
Before WWII, America was still a farm-based economy. Pre-war farmers burned alot more calories than a person living a mostly sedentary life. One study of Old Order Amish showed they average 15,000 steps during a typical day, and they engage in alot more physically demanding activities around their farms.
In the 30's, 40's, and even early 50's in the US did alot more walking and had alot more non-exercise related physical activities (even somebody working at a job sorting toothbrushes is engaged in physical activity that is burning alot more calories than somebody sitting at a desk, something the person making the video doesn't seem to understand). Adults also often engaged in more social activities, they were also more likely to attend religious services. All those are factors that tend to be associated with lower BMI.
Snack foods were not widely available, serving sizes were smaller, and food was more expensive. Food in general wasn't advertisedas much. Fast food was something people ate only occasionally, and portion sizes were generally small (my grandfather, when he went to McDonalds, ordered the basic hamburger his whole life. Today, hardly anybody buys a simple hamburger at McDonald's).
In the 1960's, BMI did start trending upward, though it didn't rise as much as it did in the 1970's and 80's.
If you really want to understand how obesity is related to the food environment, you need to look up Dr. Marion Nestle. She is a Harvard professor of nutrition and has written alot about how the financialization of our economy, as well as big business friendly deregulation starting under Carter and Reagan, has created perverse incentives to market cheap, unhealthy foods to the US population. Something that didn't exist in the 1950's, when food was mostly the product of small companies and family farms.