Why was Monica Geller overweight in her younger years?

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.