do you mean the extensions forward of the hinge line on some rudders and elevators? (rarely on ailerons But I've seen them there too.)

If that is it they are aerodynamic balancing areas... they help reduce the possiblity of flutter and take some load off of the servo.

They aren't really to increase the control surface area... you can do that just by having the hinge line a little further forward. This can increase the area by the same amount and in some cases is easier.