I'm not totally sure. If a fat person was bitten then they would stay that way, right? But yeah, once they become वैंपायर 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 वैंपायर lure in their victims they are perfect and everyone falls in प्यार with their beauty so therefor would do anything for a vampire या go anywhere with a vampire!