From where this statment come from?
More Silicon longer life of engine, more silicon more stable geometry, more silicon better expention rate-everything is more-only one thing is realy difficult-to put more Silicon to Al alloy and have piston not bridal ( too much silicon piston brake so easy). A lot of developments was done to find right alloy with great ammount of SI and keep very strong.
BTW, OS pistons doesn't contain that much Silicone as others.