Maybe because the bigger you are, the more target area an opponent has to hit in point sparring. Your arms can cover only so much space. It doesn't matter if the hit will faze the person, all that matters is that you hit legal target area before the person can react to block it.
By THIS GUY I didn't mean you :) I meant the ATA instructor that I heard telling a 13 year old BB that it was not advantageous in order to do so and I was wondering why HE thought that. I really have no idea why he thought that I was just trying to think of some reasons HE would have for that.