It's not because she couldn't stop shoveling food into her piehole:

Rosie O'Donnell blames fame for making her fat. O'Donnell writes in a column in the gay magazine The Advocate that fame robbed her of her "yellow," which is her way of saying her energy or happiness.

O'Donnell says she got "bigger and sadder" as she became more "starved for yellow." O'Donnell's solution was to eat more and more. O'Donnell says the bigger she got, the angrier she felt.

Maybe she'll sue all of Hollywood, since no one is responsible for their own actions nowadays.