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.
“Most kids”
Even if we stipulate everything you say, that still doesn’t account for adults.
“they didn’t offer “super size” or “jumbo.”
The first MacDonald’s in my area sold burgers by the bag, five to a bag.