What is the diameter of your overflow pipe? Just one drain?
What type of pump and flow rating?
The guys above almost certainly gave you the answer, but it's always nice to know the specs so you can determine if it's a "normal" issue or something out of the ordinary.
Yeah, just a case of the pump being slightly too big for the overflow...easy fix...get a good ball valve, and cement it into place, you can run a hose temporarily from the pump to the tank, while the cement dries...
and if you want to go really nuts you can pick up a "T" as well and allow the pump to pump full force but just divert some of the water back through the sump again