Think Shazaam covered this a while ago.... it's something along the lines of you're pulling the lever the same amount, so the same amount of fluid is pushing the oversized (key work here) piston, thus the piston doesn't move the pushrod as much as the OEM version, thus the downside is disengagement/clutch dragging etc.
Could be something else though, air in the system, leak somewhere, try adjusting the span on the lever outwards, which equates to more fluid being pushed at the master cylinder, and the knock on being the slave pushes the pushrod further.