My theory, with no citations except anecdotal evidence:
The rich of the 70s adopted the “can’t be too rich or too thin”. Middle class emulated them. The poor couldn’t afford to eat well so they stayed thin.
The rich of the now are still thin. The middle class (whatever remains of it) still emulate it, mostly. The poor can find filling cheap food like chicken nuggets and beef slime burgers, and other chemical-laden, highly processed foods that came from technology being able to extract every bit of profit from crops of corn and soy.
I notice a much higher percentage of overweight-to-obese people in the public now.
Anecdotal evidence: We went to Velvet ice cream’s festival last summer. Granted, it’s ice cream, but almost everyone there was overweight-to-obese. Maybe 10% of the festival-goers were slim to average, 50% overweight, 40% obese. Most of them did not walk far before having to sit down and pant, and most of them dropped out of the tour when presented with stairs.
So:
More cheap processed foods made from things our bodies might not be able to use properly, larger portions, engineered snack foods that make you want more fat-sugar-salt, less exercise built in to our daily routines, no free-range kids getting hours of running, hiking, climbing, etc - all adds up to a weightier society.
