I think it largely has to do with not being as active. We have a lot more desk jobs now than we had in the past. In the past people didn't have to go to the gym to not be overweight, they burned it all off when they were going throughout their day.
But even "blue collar" jobs require less labor than before
I really think it has less to do with that—though not nothing to do with that—and more to do with economic and convenience factors that lead to more people choosing very cheap and awful food in excess, consistently, while not getting any exercise