When you sit 9 hours a day you need to hit the gym or diet. When I was waiting tables and bartending in college (and a lil after) I didn't need to workout -- a doubleshift on my feet kept me very lean.
When you have money and a real need for activity then yeah the gym lifestyle becomes a thing.
Almost everyone over 30 has some level of gut now, even those considered comparatively fit and thin. This wasn’t normal decades ago, but is normal now.
How often do you see people shirtless? Clothes hide a lot of bellies. Having muscles, being a jogger, eating salads or riding s bicycle is completely consistent with abdominal fat.
So, what standard are you using when you say urban people now are likely thinner than rural people in 1990?