Earlier this evening, while having dinner with an Indian friend who came to Hong Kong for a short business trip, she asked whether we Chinese like to order a lot of food, I told her that this is related to the Chinese mentality of face. In having extra food left on the table, they would like to give those they are having the meal with an impression of abundance. Having face is a bigger concern than not wasting food.
Some Chinese people would try to ease their conscience by cramming the surplus food into their stomach. While making them feel horrible physically, this action creates a false impression that no food is wasted. The truth remains, however, the surplus food is never needed and shouldn't have been ordered in the first place.
For rational people, this face-saving thing is rather hard to understand, but for the Chinese, it is something that they would morbidly hang on to.