Their biggish crowds are negated by the fact they charge next to nowt for the fans to get in and flood the place with free tickets. For example their gate receipts are far smaller than the likes of Southamptons are as shown in the last set of accounts.
Regarding the free tickets thing, a mate of mine works at Newcastle Uni and found out recently that the uni gets a load of discounted tickets for home games that uni staff can apply for. Never knew we did things like that, otherwise I'd have taken advantage of it whilst I was working there.
I'd imagine every top football club in the country does things like this, just not on the scale the mackems do it.