I’m not sure what the relationship is. I know that the whole game is about creating the most "anabolic environment" that you can though. This can mean eating lots of protein, using a few supplements, taking steroids or lifting heavy...
I don’t understand the chemistry but lifting creates an environment that is more conducive to muscle growth. Working BIG muscles is even better at creating that environment. Doing legs is the best (natural) way or so I have heard.
All that is hear-say, I can't offer any research to prove my point. What I can testify to, though, is that whenever I hit a plateau (couldn’t loose more fat, couldn't grow my chest/arms/back) busting my ass on legs is always the ticket to making the rest of my body follow along....
BTW: doing legs SUCKS, its pure hell. TORURE in fact. God I hate doing legs. Every day I do 'em I feel like I am about to collapse and die for the rest of the day. God, I love doing legs....