My grandfather would say "take all you'd like, but eat all you take" which had a similar "plate clearing" idea, but was also more responsible (likely due to growing up during the great depression).

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.