The job of a stripper is to separate the escaping air from the pellet. Imagine the conditions at the instant the back end of the pellet leaves the muzzle. This is when the air starts to try to rush past the pellet that has blocked its path all the way along the barrel. If there is a gap in front of the pellet before the head end enters the stripper cone, what is to stop the air rushing round the pellet and getting there first? When the head of the pellet enters the cone (which has a clearance bore but is still quite tight to the pellet diameter) the only convenient path for the air is to expand radially and flow around the outside of the cone and out through the vents or ports.
So I think that the gap can easily be too large for efficiency, just as it can be too small.