I'd say the key difference between a 'spotlight hog/main character syndrome' and 'a player who RPs a lot in a group full of quiet people', is does the player also start talking/engaging whenever the spotlight is on the quiet player.

Because I think that's the real rub; that difference between a player 'dragging' the less engaged players behind them (good and fine), and a player who can't allow themselves to step back and let someone else be the main character for a single scene.