You eat enough to maintain your bodyweight but also enough to adequately recover from workout to workout. Also, many if not most lifters train a few kilos above their weight class in order to lift heavier in training and create an awesome training effect. For example, from one of the announcers at the Sydney Olympics, I heard that Dimas weighed WELL over 85kg, close to 90+ the week before the competition, and had done 185/220 in training if i recally. Dimas was an extreme however, because most lifters I know will weigh up to only 3kg more at the most.
One thing that I have noticed that is in order to adequately recover from heavier workouts I must eat more, yet I will not gain weight. Also, the stronger you get, the more you have to eat to maintain weight and recover.