I grew up in 80s rather than 70s, and we were all very thin. My grandma cooked most of our meals and she was an amazing cook - but it involved lots of fat, meat and carbs! On Sundays alone we’d have a full English breakfast, then a Sunday roast with two or three kinds of potatoes and Yorkshire puddings. There’d be a home-made pudding afterwards, made with and served with double cream - sherry trifle, or crème caramel. Then for tea there’d be homemade scones, a cheese board, and cream cakes for afterwards.
Alright, we didn’t eat like that every day, but evening meals were similar large portions of heavy, carby, high-fat food, followed by pudding. It can’t just be that she cooked from scratch and used less processed food, because I do the same, and would be twice the size if I ate like I did then. Can’t just be age, because there were 5 generations living in our house and we were all thin. The adults smoked, but the kids didn’t.
All I think is that the adults didn’t snack (kids were allowed a couple of biscuits or a small slice of cake after school, but that was all); only ate sweets once a week (Saturday treat), abd takeaways were unheard of - maybe once a year we might get fish and chips, on a bank holiday.
Still though. We were very slim, considering what we ate.