How do boxers gain so much weight from the time they weigh in to the time they fight?

Boxers purge them selves of all excess weight before a fight i.e. water and waste weight. They will usually start a diet and regime of coffee (laxative) while spending hours not only working out but also sitting fully clothed or in a rubber suit in a sauna to rid themselves of all the water in there system. After making weight they will slowly rehydrate there body as well as eat and that is why you often see a drastic weight gain from weigh in to fight night. They buff up, that way, they become heavier.