Have to agree with Onion with regards to area population and airport pull in that saying Doncaster is bigger than Newcastle is a little skewed. The city centres of Newcastle and Gateshead are separated only by the width of the Tyne a couple minutes walk. I think you'd have to include Gateshead’s population with Newcastle and that would push Newcastle up with Leeds and Manchester.
Doncaster is pretty much a stand-alone centre, but the road, motorway and rail link network with Sheffield, Rotherham, and Bassetlaw are very good, and can be reached in minutes.
The one thing Doncaster (as a stand-alone centre) has got going for it, is its enviable geographical location. It’s set to become a logistic and transport communication hub, which will see unprecedented and continued growth, and a user friendly town/city for airport passengers and freight.
A good indicator of measuring success rate is on the average passenger per flight ratio. Using the CAA figures, DSA is up there with the big boys at an average of 109 per flight, and ahead of the next in the (said topic) list, MME (86).
I think most of the above airports will grow, but problems may arise when funding/finance is needed to upgrade terminals, lengthen runways etc in order to meet demand.