QJ, if the air cell is too small then humidity needs to be lower. Not enough moisture has left the egg. You need to encourage more evaporation from the egg. Lower humidity encourages more evaporation.
If the air cell is too big that means it is losing too much moisture too fast, so you raise humidity to slow down the moisture loss.
In a while I'll put up the diagram I have used to monitor moisture loss during the hatch. It's from one of Holderread's books. When you candle, you match the look of the egg to the diagram.
Eggs from different ducks lose moisture at different rates, I have found. It's got to do with the thickness of the shell and the mucous coating they put on it at laying time.
Interestingly, the duckling is producing water during the incubation - as it uses up the yolk a by-product is water.