Just looking at the most recent stats from the ABS, the Sunshine Coast is now bigger than Woollongong, The Central Coast and Townsville and is growing at a significantly faster rate than Canberra, Newcastle and the Gold Coast (and every capital city, for that matter).
If it continues at this rate you'd have to imagine a token 2-3 Dolphins games per year will be insufficient before long and it'll be demanding its own team. So this stadium upgrade is welcome.
Rates of growth don't mean much in real terms in this context. At current growth rates it'll be a very long time before the Sunshine Coast's population catches up with the Gold Coast's or Canberra's, let alone the major capital cities, if it ever does.
The more interesting difference between three are the demographics of the immigrants to each. Put simply, mostly retirees from Victoria and NSW are immigrating to the SSC and GC, while the average age of Canberra's immigrants tend to be much younger and more diverse. The point isn't to make it some sort of competition, but to point out that the average retiree from Victoria and NSW isn't very likely to be interested in supporting a Sunshine Coast NRL side.
Besides, projected growth rates are going to be more unreliable than ever if even conservative projected temperature increases caused by climate change are accurate. Who really knows how things will turn out on that front, but as things stand it's as likely as not that large parts of the northern and central part of this country will become increasingly inhospitable, and that the population growth in those regions will significantly decrease, if not invert, for everywhere north of Mid-North Coast.