I think the reason they are big is too give loads of throw (like 70 degrees) but with less drag than a smaller surfaces
wouldn't a bigger surface create more drag? I know what you are getting at but I just think this might sound wrong to somebody who doesn't know about it, I'm not an expert at the subject but i imagine the big elevators are simply to increase tail moment, and if anything, create more drag.
Also, a big surface moving a small distance would have the same effect as a small surface moving a big distance, so less movement is needed to get the desired effect.