He’s Too Sexy for His Fat is episode 17 of family guy season 02. It was originally aired on June 27, 2000.
When Chris is afraid to remove his shirt at a local hotel pool because he’s embarrassed about his weight, Peter tries to convince Chris to get liposuction, but likes the idea so much, he gets it himself. After surgery, Peter looks thinner and everyone treats him better. This sends him back for more plastic surgery until he realizes looks can be deceiving.