In the early seasons of Friends, none of the main characters were depicted as overweight. But, throughout the seasons, the writers put a lot of emphasis on weight.
There are several blogs which focus solely on Chandler’s extreme weight gains and losses: http://www.tv.com/shows/friends/forums/matthew-perrys-weight-changes-186-1011849/
Chandler Bing (Matthew Perry) starts off the season as a very jumpy, energetic, and skinny man. Few people know that he was this thin because he has a pain killer addiction after getting into a skiing accident in 1997. He was very thin because he was addicted to Vicodin. He then became heavier in seasons two and three because he became addicted to alcohol. Chandler’s character moved to Tulsa in the seventh season even though he was married to Monica, who stayed in New York. The writers had to do this because Matthew Perry was checked into rehab at this time. In season two, episode seven, “The One Where Ross Finds Out”, there was a whole episode on Chandlers sudden weight gain. Chandler’s friends make several jokes about Chandler getting heavy, so Monica takes him on as a project because she is unemployed.
The gang often brings up how fat Monica used to be when she was younger. They told stories of her breaking chairs, eating whole family dinners so that the family would go to bed hungry, and her having to make up a boyfriend in high school because the boys thought she was too fat. Ross reminisces the time when Monica was sent to her room and she ate macaroni off of a spray painted Christmas wreath that she made for school. He also talks about how the family couldn’t afford to have a clubhouse because they had to pay for their dog’s surgery since Monica sat on the dog and crushed it. The writers and directors have several “Fat Monica” scenes where Courtney Cox is shown wearing a fat suit and constantly eating in these scenes. Fat Monica is a compete mockery of overweight people. They make it seem as if Monica cared about nothing other than food. Monica went on a diet and shed pounds when she was 18 after she overheard Chandler call her “fat” at Thanksgiving. She also heard Chandler say that he didn’t want to be stuck in the house with Ross’ fat sister. This really made Monica think about her weight. When the gang was watching Monica and Rachel’s high school prom video, Joey Tribbiani shouts, “Some girl ate Monica!” Any episode which depicts Fat Monica, her character is always dancing in a mocking manner with food in her hand. http://www.youtube.com/watch?v=fn1ZOoSfO_A The gang also makes jokes about how her marching band costume in high school had to be custom made. Ross once told the gang that he had to see a therapist because he had reoccurring dreams that Monica was trying to eat him. Ross also tells the story about the time Monica almost got kicked out of fat camp because she was trying to eat a squirrel and got caught in barbed wire. When Monica was not invited to her cousins wedding, Ross tells her that the last time her cousin saw her, that she could have turned a chair into kindling.
In the episode, “The One That Could Have Been”, the writers developed a plot scenario if the things would have happened differently in their lives. The writers thought it would be funny if Monica had never lost weight. In this episode, she is a 32 year old virgin who is constantly eating.
There are several blogs which discuss Fat Monica at length: