It's not the size of the plate, it's the content.
In the 70s people had much worse standard of living than today. Vegies and protein cooked from scratch were staple diet. Not much money left for chocalates, crisps or chips everyday.
Today, with both parents working, expensive housing and lack of resources multiplied by cheap sauces (full of sugar) pizzas, and even cheese with content with gluten, it's no wonder our western societies are getting obese. Fresh vegies, fish and meat are expensive.
I made today a salad I love for dinner. Boiled eggs, potato, carrott, lettuce, tomatoes, tuna, corn kernels, apple, spanish black olives, cucumber and extra virgin olive oil. Yummy but not cheap. Many families can't afford or have the time today to eat healthy