According to chapters 5-7 of The Omnivore's Dilemma, what is the psychological tactic behind supersizing, and why does it work?

The concept of supersizing anything is the brainchild of David Wallerstein. Wallerstein was on the board of directors for McDonald's. Before that, Wallerstein worked for a chain of movie theaters. His goal for both jobs was to sell more product; however, he realized people rarely went back and bought a second soda, popcorn, fry, or the like. The book says that purchasing the second order rarely happened because it made people feel "piggish" or glutinous. Wallerstein discovered he could convince people to buy more, a lot more, if it came in a single order. That order could be huge too, and people will order it and eat all of it. It doesn't matter if the person is full. That person will eat all of the food in the container. The text says that this is an evolutionary trait. We, and other animals, are psychologically geared to eat everything that is on the plate or in the box because, deep down, we feel that we might need those stored reserves in the future. We are driven to feast in order to survive the potential future famine.