Re: seem to be having much the same rates of obesity.
What we do have in common is a rising standard of living and much less need to engage in physical activity.
Not data, but anecdote that I expect with a bit of research could become data:
When I was young in my last year of high school I topped out the scales at 133 pounds. When I was in college I added few, but still stayed under 140. For a 5' 6" guy, those are about right. These days I fight to get under 200. What's changed? Well in college I was still walking all day to classes and had a job that sometimes required me to get from one corner of campus to the opposite in less than 20 minutes. When I took the elevator is was usually because I was hauling equipment, the rest of the time the stairs tended to be quicker. Campus was about 1 mile on the long side, half on the short one. Now I drive to the train station which drops me off yards from my office building. I take the elevator to the 9th floor. Then I reverse the process to go home. Where after starting dinner I plop down in my recliner until it is ready, then eat in my recliner. Then I go to bed. Commute plus work runs in the neighborhood of 12 hours, so I don't feel much like doing anything else when I do get home. Weekend are chores on Saturday, which again mostly involve the car for grocery shopping, then laundry including a couple runs up and down two flight of stairs.
In short, I live an essentially sedentary life and I know it. I've tried a couple times to break out of it, but found myself getting sick when I did.
My roommate has a similar type of schedule, except she drives to work. Still mostly sedentary. From time to time she signs up for a weight watcher class or aerobics or some such which lasts maybe 6 months. No more than twice a week for the classes.