If the air cell is too small, that means the humidty was too high in the incubatore.
The eggs lose moisture as they incubate. As moisture goes out, the air cell gets bigger. That's why you can find all those diagrams online of how big the air cell is supposed to be at what stage of incubation.
If the humidity in your incubator is too high or too low, the eggs lose moisture at the wrong rate, and the air cells are the wrong size. If you keep the incubator a bit drier, the eggs will lose moisture faster, and the air cells will be bigger at hatching time.
During hatching, they need a high enough humidity to keep the duckling from sticking to the shell. Do not reduce humidity in the hatcher. But for the ones still in the incubator, you can change the humidity up or down until you get it right.