These threads come up every now and again and go exactly how Mawkish stated. They're not about anything other than moralising and being judgemental, hence, you never see threads such as 'Everyone wore hats in the 1950s-60s' it's always about size.
I think perhaps people are confusing observations with moral judgement here - on both sides of the argument really.
Some older posters are saying "this is how it was in the 50s/60s/70s." But this is not to say that people were morally superior then, just that this is how it was. Take cooking from scratch. People did it because they often had no other choice. It's not because they were all super keen on feeding their family healthy, freshly prepared food so much as the alternatives didn't exist or, if they did, they were unaffordable. The first "ready meal" I remember seeing would be in the late 1970s when the supermarkets started to sell pizzas, but they were a rare treat because my Mum had a fairly tight budget. Dad grew all of our veg - but this wasn't because he was some kind of veggie eco-warrier but because we lived in the country, had a big garden and that is just what people did to make ends meet.
Perhaps we did walk more rather than using a car. But again, this isn't because people back then were naturally less lazy. No doubt if more families could have afforded a car, or afforded two cars one for Mum and one for Dad, then they would have used them.
So I think saying there were fewer fat people in the 1950s/60s/70s is fact.
And that this was largely due to lifestyle differences is also true.
However, the older generation shouldn't feel superior about that because it wasn't a moral choice it is just that there weren't many alternatives to that lifestyle.
Similarly the younger generation shouldn't be so snippy when it is observed that there were fewer obese people then because, in general, people ate less and moved about more. People did eat less and move about more but they did so because they had no other choice.