Personally, I largely blame our learned culture of referencing “children in Africa”. Want to fuck up a child’s eating patterns for life? Force them to clean their plate.
I grew up in France and I have always cleaned my plate as my mother told me. In my familly, it's the polite and sensitive way to eat. Same expectation when visiting friends. Really few are overweight.
Since I moved to Japan, I learned that here too food is treated with the same deference : "you shall not let a grain of rice left in your bowl" is basic table decency. Few overweigh people here either.
Both France and Japan have their food recognized as UNESCO cultural world heritages. In both countries people LOVE to eat and hate to waste.
Maybe people should treat their food and themselves with respect and only put on the plate what they should and can eat?
I've told my children that they should usually clean their plate. Not always, because noone's perfect, but if they make a mistake often it's time to think about what they put on that plate.
The issue with the "children in africa" thing is that someone else is setting the portions for you, I think. Everywhere I go, meals are just too big! I feel silly only eating half of a meal, especially if I don't have the ability to save the rest for later.
Portion sizes, like everything else in America, are just bigger.
We wanted to try to help local restaurants, so we get take-out food 2 or 3 times a week. Since we're not eating in the restaurant, all the food stays at the house-- we get lots of leftovers!
I frequently tell the kids to order what they want, but eat all of it. (If not today, finish it tomorrow.)