Personally, I’d see that as overly complicated as well as oddly judge-y about fatness
Ultimately, such a feature could (should?) be reserved for a more "realistic" mode, but I don't see this as judgmental any more than...
Starvation/malnutrition resulting in a slimmer character.
Sleep deprivation causing a character to appear haggard.
Poison/disease crippling a character and/or rendering them visibly ill.
Certain moral/ethical choices shifting a character's Alignment.
These are all instances of a character suffering the consequences of an adventuring lifestyle.
I'd also point out that a flaw of a system where a character built to appear overwheight would get thinner and more fit or scrawny characters getting bigger and more fit would be that it would lead to all characters ending up with the same build. It would take away the variety that's the point of rpg character creation. I don't think it would be all that interesting as a system as a result. The closest analogue we currently have is how in Witcher 3 Geralt's beard would grow over time. That works because it's a way to personalize a character that's already very set and default. It made him more unique from player to player rather than less.
Not necessarily. An elf that is, for example, an expert swimmer and archer is going to look different from the maul-swinging human pack mule of the party.
