I too used to think like Barracudahockey, until a little more thought took place...granted the servo disc promotes a non linear response to the pushrod, but the horn on the control surface also move through an arc, centred on the hinge line. The total response of the system is still linear, ie the surface moves proportionally to stick movement. Expo, as advised above, provides a non linear surface response to stick movement either from the middle or the end depending on requirement.
Evan, WB #12.