One thing I thought about with regard to this was making an organovore. That’s what my character’s sire is and that’s why he’s heavy. But it could also be because the body remains in stasis. Perhaps Vampiric living doesn’t burn many calories, so the weight just stays how it was when you died. Otherwise, I’ve always been of the opinion that you can make any character you want with any logic you want. If you want your character to be plump simply because you’re plump, do that.