“I’m not referring to the 1950s and 1960s”
My point is that we ate a lot and exercised but little in the 1950s and 1960s, but obesity was still rare.
Define "exercise." Back in the 50s, 60s and even much of the 70s, people lived much more active lives. Maybe most people weren't at the club pumping iron, but they weren't as sedentary. Most kids in my neighborhood were forced out during the day to go play. We rode our bikes, built forts, played war, and played tag, football and baseball. Now many kids are either watching TV or playing video games.
At school, you were ran around during morning and afternoon recess, before or after lunch, and during gym class. Now, some elementary schools are removing gym class. During recess, kids are banned from a lot of the games we used to play out of concern over "safety."
It's not just the activity level. Compare the fast food portions from then to what's offered now. At my house, McDondalds was a "treat" and they didn't offer "super size" or "jumbo." Besides the fast food, we've got all kinds of junk food, readily available in portions that would never be considered before.
But wasn't the eating constrained to mealtimes? When I lived in France in the early 80s, I noticed that the French were simply NOT FAT, although they are a lot for lunch and dinner. (Breakfast was often just coffee and a cigarette for them...)
I remember hanging out with a French friend and she would say "Oh, I am STARVING to death!" While in her home with a kitchen full of food. She would not consider going to get something to eat, like we Americans would, because it was NOT YET MEALTIME! She waited! Who does that here??