I'm not totally sure. If a fat person was bitten then they would stay that way, right? But yeah, once they become Vampire can't gain any weight. Though theres nothing that says they all have to be skinny.
i think they would turn perfect like it says in the books. thats how Vampire lure in their victims they are perfect and everyone falls in Liebe with their beauty so therefor would do anything for a vampire oder go anywhere with a vampire!