Ok toast might not be the most nutritious food but it's not making people fat.
One piece of toast won't make you fat, but the fact you have not had enough nutrition in the morning may mean you are very hungry later. This in its turn may lead to a latte and a cupcake elevenses; a chocolate bar.
In the 1960s and 1970s most people had breakfast with some cereal and milk and sugar (in a small bowl), but often too an agg and bacon, or baked beans, or kipper. You would not be hungry for hours and were well nourished. As another poster said, people would have lunch with some meat, some potato and vegetables and time to eat it in without stress...who has that sort of nutrition for lunch now? An evening meal would be similar.
No one missed meals. Tea in the middle would keep people going with a warm drink and one or two biscuits (none of which compared to a 6inch wide cookie) or something like a small piece of sponge cake.
The balance and the quantity was different.
No one ate just bread, giant bowls of cereal, pasta, enormous pizzas on a regular basis, or foot long "french" bread sandwiches filled with sugar sauce and grease and with side order of chips. Cafe binge food did not exist. Coca cola or lemonade were an occasional treat - the giant binge bottles of fizzy sugar drinks and diet drinks (the diet stuff causes cravings) were not standard.
Meal deals are contributors to obesity. Binge packages in cinemas replacing small individual portions of sweets or popcorn have changed culture too. Those bumper bags are evil. "For sharing" is a euphemism.
Television advertisements show scenes like the one where people on the sofa are arbitrarily shovelling giant chunks of greasy roast potato into their mouths while watching the television mindlessly. These used to be a small treat that was part of a Sunday lunch or a holiday meal, with meat and other vegetables.
Lives were previously physically more active too as other posters have said.