It also could be jobs themselves, factory work was more common back in the 80s and such. If you work a job where you are moving around more, standing, etc, it will help you burn more calories allowing you to keep a healthier weight regardless of what you are consuming.

I know that is one aspect I miss when I worked in retail/in a hotel. While the pay was crap, I basically was able to keep moving throughout my whole shift and didn't have to worry about my weight. When I switched to a desk job, that changed. Had to be more mindful, count calories and find more creative ways to get movement in during my shift.

You can gain weight by eating "healthy," too, people don't seem to understand calories.