This, they have pushed their bodies to be able to intake massive quantities of food. For people who get big, and then actually lose the weight it's a common...observation that they can't eat as much as they did before. They feel fuller faster on less food. Their stomach adjusts to going back to normal portions. Like others have said, they also get used to feeling ill. That becomes the normal, so they don't recognize that what they are feeling related to how much the eat, and what the eat isn't normal. They don't recognize that their body is sending signals that something is wrong.