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.
That just means that all elves end up looking the same, all humans end up looking the same, etc. It's just determined based on race. I can't imagine a game being so specific as to devote the time and money necessary to properly model the intricacies of how various forms of exercise and activity impact physical appearance and weight loss/gain unless that were all the game was about. Otherwise yes, you'd get a system where no matter what you start with, your character just ends up defaulting to a median character build set as 'fit'.
