There is of course an alternative paradigm - maybe one of the factors at play (and there will be a number of factors) is diet. Maybe less fat - replaced by carbs - is the problem
Anyway - here is the BBC story
Despite fewer visits to gyms and a love of high-fat foods, people in the 1960s were slimmer simply because they were more active, the government says.
Rates of obesity in English adults have risen from 1-2% in the 1960s to around 26% today, figures show.
......high fat foods like condensed milk and cooked breakfast were popular ..........
Of course there is a role for activity and movement is vital.....but see how fatty food is seen as a problem, not a reason for the lower levels of obesity