It is all about the way you look at geography.
These are urban areas (comparative to the sprawl that we have here in our cities even if there is greater separation between towns). European counties are small compared to the area of Australia:
Greater Manchester area = 1,276 km²
Greater Melbourne area = 1,276 km²
Barcelona Province = 100 km²
Look it up if you don't believe me.
So Greater Manchester has 3 EPL clubs ?? Melbourne has 9 AFL clubs ... plus one in Geelong, which could be considered to be part of "Melbourne Province".
I am sure that a "Melbourne United" AFL club would easily be a greater supported club from a local perspective than Manchester United.
I understand what you're saying, and believe me as a Melburnian I love the fact that we can sustain nine professional football clubs.
My issue with your original post was that it implied that Manchester United and Barcelona are representative of Greater Manchester and Barcelona Provence respectively. Sure, they are the biggest clubs in their regions, but they are not only the teams competing at the top level. Do you really think a fan of a consistently competitive team like Bolton is going to travel to Old Trafford to watch United, just because he/she also lives in Greater Manchester? Ditto for supporters of Wigan, who have seen their side climb three divisions in the past five years? And what about Manchester City fans, who hate all things United with a passion?
As for a `Melbourne United' footy team being better supported from a local perspective than Manchester United, you're probably right, as it's United's global appeal that makes them such a powerful institution. A fair percentage of their season ticket holders don't live in Manchester.