Good points.. and AFL had an easy choice for expansion, because apart from Geelong they just don't have mid-sized cities in their heartland like how the NRL has Canberra, Townsville, Newcastle & Woollongong. So putting 2 teams in Perth & two teams in Adelaide was a no-brainer.
Sure, if the NRL ditched the Raiders, Cowboys, Knights & Dragons we could have Brisbane 2, Perth, Adelaide & NZ 2 (probably the 4 next desirable expansions) and STILL have only 16 teams but that's never gonna happen.
[Edit - under that scenario, you could then add a 2nd Melbourne team and a 2nd Perth team, and have a very similar footprint to the AFL with 18 teams - same number as the AFL.
The only clubs the AFL would have that NRL wouldn't would be a 2nd Adelaide team (unlikely) and Geelong (even more unlikely, given this scenario ditches Newcastle, Canberra & Townsville)
The NRL will never leave the mid-size cities, but just goes to show the difference in approach.]
It's a strength of the NRL that it represents the heartland so well, but it also means the competition will just HAVE to be bigger than AFL to get a similar reach - unless Sydney is restructured.