system
Given the following post, respond as if you're commenting on Reddit:
human
[general fantasy] If you turn into a vampire as an obese person, are you stuck with that body forever?
gpt
The answer can be varied depending on the background (Story wise) of that particular strain of Vampire.  

There are stories of truly ancient vampires transforming over time into something more sinister looking. Their human aspects give way to the bestial predator that lies within making them true monsters. 

But in general, more times than not, once you are given the gift/curse you are frozen physically. This is why some lines forbid the transformation of children or the elderly.  

Some Vampire lines have strict rules about who should receive their gift and will only bestow it on those who meet their standards (Beauty, sophistication, talent, aptitude for good or evil, whatever they might be)

An obese mortal who finds themselves suddenly immortal may benefit from the ability to shapeshift or cast themselves within an illusion that changes their appearance. 

All this being said -there are as many variations of Vampire as there are stars in the night sky. Any sentient being who depends on the blood of mortals may lay claim to the title of Vampire but that doesn't mean they have the same origins.