Even if the throttle were wide open the engine in the suburban will still be working harder than that in the corvette due to the weight of the car.

Therefore if a standard size control surface is used, a 4 pound plane would need less servo torque to move the suface than a 12 pound plane. Due to aircraft inertia, and the larger force required to move a larger object.

So the size and torque of the servo is proportional to the size of the aircraft and the size of the surface you wish to move. So if the plane is large, (with a small elevator for example) you only need a medium torque servo. However if the plane is large and has a large elevator, you need a high torque servo.
On the other hand if you have a small plane with a large elevator you can use a lower torque servo.

However this is where we runn into the issue of servo speed, and how long the servo takes to move the control surface.