- Why was Monica Geller overweight in her early years?
- How did she become normal weight?
Monica was fat in her younger years because she ate too much.
In The One Where Rachel Tells... Ross says:
I grew up with Monica. If you didn't eat fast, you didn't eat!
In The One With The Pediatrician we learn that Ross used to go to a therapist, because
Monica: ... he used to have this recurring nightmare. It really freaked him out.
Rachel: Wow, what was it?
Monica: That I was going to eat him.
In The One With Phoebe's Cookies it is revealed that Monica was once sent to fat camp:
Ross: He still tells the story about how Monica tried to escape from fat camp.
Monica: I wasn't escaping!
Ross: Then how did you get caught in the barbed wire?
Monica: I was trying to help out a squirrel.
Ross: You were trying to eat it!
Monica lost the weight because of an incident during Thanksgiving 1987, when she overhears Chandler say to Ross:
I just don't want to be stuck here all night with your fat sister.
That prompted Monica to lose weight, which she successfully managed by Thanksgiving 1988.