I've done this quite often. Yes, they let a smaller (less powerful) servo do a big job. I did find that you want to limit the size to 10% of the moveable surface area though or you will get hunting around neutral. I seem to have best results when the tab always stay in line with the neutral position. This is easy, just make the distance from the horn to the pivot points equal at both the fixed and the moveable horn. On excessively large surfaces, it also helps to statically or mass balance the moveable surface, especially on ailerons or a hard landing can cause the excessive momentum of the large surfaces to strip the aileron gears.