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.
