Why did Matthew Perry gain so much weight in season 6?
Matthew Perry, who played Chandler was addicted to drugs and went through rehabilitaiton to get rid of the addiction. This was why he kept gaining and losing weight during a few seasons.
Why is Chandler so thin in season 7?
In 2000 he was hospitalized with pancreatitis as a result of his addictions. During that summer he lost 20 pounds, appearing noticeably thinner during the 7th season premiere of Friends, in a scene that continued directlyvfrom the 6th season finale.