I was born in the late 50s, and started school in 1959. There were no fast food outlets, apart from the fish and chip shop and that was a weekly treat - if that.
There was not the huge range of fizzy drinks, crisps and snacks that there are nowadays.
Also, for women who stayed home after marriage housework was much more labour intensive than it is now, lots of floor scrubbing, not many washing machines, wringing sheets through a mangle, no convenience foods to pop in the oven.
Plus there were fewer cars, nearly all children walked to and from school.
So I can honestly say that there were very few people of the size you see nowadays, a size that seems to becoming normalised.
Obviously there was the occasional tubby child, or older women looking a bit plump.
Children played out on the streets and in the fields, skipping rope, ball games, running around, cycling - they had no computer screens to sit in front of for hours on end.
So yes, people were much slimmer in those days - for all the above reasons.