Until modern times dieting to lose weight was almost unheard of. Food wasn’t as easily available as it is today (at least in the US and Europe) and most people were much more concerned about getting enough food so as not to go hungry. 

What we call overweight these days used to be thought of as a good thing something only the rich could be. Everyone else was skinny because they couldn’t afford to eat more. We don’t know exactly when weight-loss programs began to be popular. We don’t know who was the first person to say “Oy vey I have to go on a diet!” but it was probably sometime after the invention of matzah balls.