ET's comment about population growth is a limited view, because population growth is occurring all over the city and you cannot make sweeping assumptions that more population means more football support. Potential is there yes, but if an area doesn't follow football, it might not matter how many thousands of people live there. For example one of the strongest drivers of Western Sydney population growth is immigration, and there will be a limit to which you can turn new immigrants into rugby league supporters, particularly those from countries with no RL background at all.
The Top 10 local government areas (LGA) for predicted growth (% change) to 2036 are: Camden, Parramatta, The Hills, Liverpool, Botany Bay, Strathfield, Sydney, Burwood, Blacktown, Wollondilly. The Top 10 when taken as pure headcount: Parramatta, Blacktown, Canterbury-Bankstown, Camden, The Hills, Liverpool, Sydney, Cumberland, Penrith, Campbelltown.
http://www.planning.nsw.gov.au/projections
So out of 20 slots, the Macarthur is represented 4 times, and "the Macarthur" cheats a bit because it's the combination of 3 LGAs - Campbo, Camden and Wollondilly. I mean can Tigers really lay claim to Wollondilly when most of the towns in that LGA are half as far from Wollongong as they are Sydney CBD?
Compare this to the Eels, they are sitting right in the junction of 9 slots, if you include penetration into The Hills and Liverpool LGAs.
It's all well and good to say that Tigers should do more in the Macarthur, but the same argument exists for any large population LGA in Sydney, particularly those not dominated by any one club e.g. Liverpool and The Hills.
In fact there is strong argument about indeed focusing on the inner city and inner-west, which take up 4 slots in the above 20, including huge population growth in Sydney council (bigger than Campbelltown as both % and total volume); and this does not include Canterbury, which old Wests Ashfield / Balmain territory sits against.