Andre Dubus, an important writer of his day, in 1986 had a life changing experience, he was hit by a car while trying to help a stranded motorist which results in his leg being amputated. He was confined to a wheelchair for the rest of his life.
As a result of his life changing accident, Andre DuBus became more aware of the suffering of humanity. He became more empathetic to people who were looked upon with a sense of pity.
I think that the author thought it was an important subject to examine how fat people are treated different from other people. He certainly had a point of experience from his own life, now that he was an amputee and in a wheelchair, he could compare how he was treated to his previous life before the accident.
His reflections in "The Fat Girl," are relevant to the individual accepting him or herself in whatever form they are packaged in, such as a fat person, or a handicapped person.
I feel that the author felt that his identity was in question because of his altered physical state, just as Louise is altered in this story. Louise seeks acceptance in her life, she is the same person whether fat or thin, just as Andre Dubus is whether in a wheelchair or walking on his two legs
Dubus wrote this story before the accident which took his leg; I know because I was one of his students at Bradford College and he read his drafts to us in our writing class. I pointed out to him that his idea of what a woman should weigh was much too low, especially considering his quite well-built frame. He was a runner, and quite muscular, but also quite burly. I don't think he recognised he was creating a manic depressive in Louise's roommate; he refers to her condition as 'malaise' without ever seeming to grasp that it wasn't just a girl being difficult, it was a mental illness.
Dubus liked women as lovers and baby-makers, but I don't think he ever truly understood them. The years he spent teaching at an all-girls college must have been a combination of heaven and hell for him. Perhaps Dubus created Louise to give a woman control over her life.