She put on the weight and started dressing and acting in a more masculine way after Sonny died. After a few years, she went for the surgery.
Her father’s death seems to have destroyed whatever psychological equilibrium she had.
Somewhere I read a little about her childhood, and how her parents lived when she was small. Not a really normal, wholesome (oops, there I go, talking about "normal" and "wholesome") childhood.
Sounds like she’s trying to replace him by becoming him.