The idea here is that proportionally, Aberdeen has a lot of untapped potential when it comes to fanbase.
A city with the population of 200k plus (not to mention the surrounding area), with no other SPL side within an hour's drive, can only attract on average ten or eleven thousand over the course of a season.
Take for example, St Johnstone, who last night attracted 5,700 to the Utd game. Take off a generous estimate of 1700 Utd supporters and a town of around 35,000, with a cluster of clubs including Dundee Utd, Dundee, Dunfermline and Falkirk within half an hour is getting 4000 fans - proportionally much more. While you could say that they have a more promising team at the moment, this is countered by the fact that the game was televised.
Hence why it is logical to believe that Aberdeen, with few local competitors and a relatively large population, could attract more fans than any other club side outwith the OF.