We ate less, we moved more.
I was born in 64. Throughout the 70s we played outside, all the time, we rode bikes, roller skated, swam, went to Brownies/Guides. Having to stay in and 'not being allowed out to play', was the dreaded punishment when we misbehaved.
I had Ricicles for breakfast, a third of a bottle of milk mid morning, a school lunch and an evening meal. Once a week on a Friday we had a packet of crisps and a glass of pop, that was the day the Corona van came round. We walked or rode our bikes to school, the bike shed was heaving. We got 5p to spend on penny sweets every Saturday.
In the 80s, I rode my bike to work, loads of us did. We met our pals in town every Saturday and again walked everywhere, we also went to weekly Jazzercise.
I get times have changed, some of it certainly for the better. But snacking and portion sizes have greatly increased - you would not have seen the baby food aisle crammed full of 'baby wotsits' in 1974. Wains can't manage the journey home from school (or a short trip out somewhere) these days without a snack, people seem almost scared to let their kids feel a bit hungry.