Rule #3: You Can’t Take it With You
The rest of your meal, that is. At a restaurant. Don’t save some of it for lunch tomorrow, because Rule #3 forbids doggie bags of any kind.
When I first observed this after eating out in Italy, I didn’t understand it. Sure, it seems to be a corollary to Rule #1 (Clean Your Plate), but a restaurant is a much less personal experience. If you’ve eaten your fill and there’s some leftover, why not just bring it with you?
While we economizing Americans think of this as simply making the most of an expensive meal, Italians don’t have the same attitude. In order to satisfy my curiousity, I did some research and figured it out.
First of all, unlike American restaurants, Italian restaurants serve human-sized portions. So it’s not all that difficult to finish your meal. Still, when I went to pizza place, every Italian finished an entire large personal pizza each.
It doesn’t have to do with the stomach–it’s a matter of pride. Taking the scraps of your meal home means that you cannot provide for your family. It indicates that you do not have enough to eat at home. This resulted from the war years, when Italians were incredibly poor. The reason it persists today has to do with Rule #4, Fare la Bella Figura (keeping up appearances), tomorrow’s feature.
When Nutmeg Met Basil 
Also, meals tend to last longer in Italy, giving one more time to eat. Is it also possible that Americans eat out more, so there is less importance put on the food and meal, and no disrespect on eating it cold (never as good) later on?
How could I forget to mention the length of Italian meals? I definitely agree that Italians eat out less frequently than Americans. Good point!
You got the point. It would be a “brutta figura”, because it would mean that you can’t afford another meal the next day. Just a matter of customs.
You’re right, I think that it’s related to the fact that the portions (especially at the restaurant, while at home they may be bigger) at the restaurants are small because they’re not intended to “fill your stomach” like when you’re at home, but they are meant to let you try new tastes (ie. something you can’t cook at home because it’s too difficult to cook).
For instance I can’t finish a whole portion when I’m in the US, because it’s just too big for me (and I’m everything but thin).