I asked this question once (if there are full meals left if someone was really hungry) and was told there's rarely much left over unless someone doesn't want a meal at all (one example is afternoon tea - I noticed on a flight back from CPH recently crew had to explain what it was and some didn't want to eat). Often by the time crew get to, say, row 8, there's also only a single choice left from a menu.

In CW you'll find some parts of unused meals are left in the Club Kitchen. Sometimes there might be a second choice from the light meal.

Confused about the question though as you're sort of assuming there's a tonne of unused meals piled up somewhere?