I think you are onto part of it. I wear the same size dress slacks and shirts I did in high school and I weigh about 20 pounds less at age 69 then at 18-19. I once was about 25 extra pounds and my BP went sky high. I feel miserable if my stomach is not flat.
That is impressive. I am probably 40 pounds heavier or more than my weight at 18 years of age. Am definitely not as active as back then. Have a desk job and watch Netflix too much. So for sure understand why America is fatter now.
When each family had their own farm land to cultivate, provided their own food, cut their own wood for heat, etc...t’was a different world but a healthier one. They were the fit and healthy ones compared to now. Society has not benefited in many ways from supposed “progress.”