Generally when people talk about 'balanced control surfaces' it is not about mass balance, it is about aerodynamic force balance. A large, balanced stabilizer has some portion of the surface in front of the hinge line, such that when elevator is applied (say down elevator), the smaller portion that goes (up) in front of the hinge line is exposed to the flow path and takes some of the load off the actuator/servo.

This is about rudders in marine vessels, but the point is the same:
http://en.wikipedia.org/wiki/Balanced_rudder