The reality TV star made the comment about the 'Burlesque' actress when she was asked to give her opinion on a photograph of the blonde beauty wearing a tight black Givenchy dress.
Speaking on E!'s 'Fashion Police', she said: "Maybe she is just becoming the fat bitch she was born to be. I don't know. She was a **** to me. And she bought my house!
"She called me fat for so many f***ing years. So you know what? F**k you! You're fat too."
It is not the first time the 26-year-old 'Changes' singer has insulted Christina, once referring to her as a "p***y".
Kelly previously said: "She's a p***y. She has a mouth on her, but she's all talk. She only does it because she always has a security guard around to punch anyone she gets in a fight with.
"[Christina] was like 'Yo girl, artists for artists, yo, why you been talkin' s**t?' I didn't understand a word she was saying. But I stopped saying things about her."