It is removing extra friction from the bottom. It is realy doesn't need to have any sealing on the bottom, so we made realy big tapper there. But it has limit, if it is too big, the piston will go out of straight line and will hit exhaust window's top line. So everything is reasonble limit. Can tell you, that one is one of the realy big secret of design, too many things are involve in this issue and a lot tries were done to find right tapering.