And what you fail to understand is that (a) not everyone in Brisbane is a supporter of the Broncos (making the eleventy million people MUST follow them) which (b) invalidates the catchment area argument that might make more sense in Sydney where the current supporter base has built out of a district competition.
Try reading my post more slowly, it might then become easier to comprehend. I even stated that was the case but overall, these things balance themselves out (number of people within the catchment that do not support the club and number of people outside the catchment that do). A very large percentage of RL fans in Brisbane would support the Donkeys while a small percentage of RL fans in Sydney would support any given Sydney based club. I'm not sure why you would expect any club in Sydney which have an average of 5.3 million (Sydney + Illawarra) / 9 clubs worth of people to draw from would outdraw 2.2 million /1 club. Sure, some people in Brisbane don't follow the Donkeys but there are also Melbourne, Canberra, NZ, GC, NQ and Brisbane fans in Sydney.
As far as your second point about origins from the district competition, the Donkeys were built as a QLD side to compete against the NSW competition. In effect their base was built so much larger than that of any Sydney club and it would make sense that they would draw strong support from a much greater area.
At the end of the day apples are being compared to oranges but you are a fool to think that it is impressive that the Broncos are getting the crowds they are given their situation. Souths and Canterbury will soon be competing with them for crowds whilst drawing from a much smaller potential fanbase.