This is because inrunners have lower torque but are a bit more efficient so it gets compensated.
I'm not quite sure that is the result of the effciency. It is I think more a result of the fact that an ourunner has an amount of slip (20-25%) and the inrunner does not have this slip. This is the reason that an inrunner feels different, the outrunner unloads, the inrunner tends to hold revs more.