I read a very interesting article awhile back about pop-ups. The author believes that because most big carp are deep-bodied (rugby ball shaped), it would be easier for them to suck in a pop-up that is presented about 15-20cm off the bottom (meaning that the fish will be in its normal "upright" position when approaching the pop-up) than it would be for the carp to have to tilt its body 45 degrees first to eat the bait off the bottom....
I gotta be honest I havent used anything popped up higher than 5cm, so wonder how true the above is... I have heard of some guys fishing 15cm off the bottom to take advantage of the above theory and they got some pretty big carp...interesting...