Food is seen as a gift from God and thus wasting it is seen as a sign of disregard for this gift. There are many Hadith of Prophet Muhammad that address Islamic dietary habits, at the heart of which is respect towards food.
“Nothing is worse than a person, who fills his stomach. It should be enough for the son of Adam to have a few bites to satisfy his hunger. If he wishes more, it should be: one-third for his food, one-third for his liquids, and one-third for his breath”.
The point of this Hadith is clear: people should eat only as much as they need, and they often need much less than they think.