You've got it all wrong.
When we say Metropolitan we are basically saying Greater. It is made up of cities that are connected to one another by continuous urban sprawl. What Poms refer to as Greater is sort of like a Metro area, except it is made up of LGAs and is classified as a County, which is something we don't have. Counties have their own government. When soapdodgers say Metro they are referring to the county area plus regions that people travel from to work in the greater area.
It says so on PR's link.
If you want to compare apples with apples, then the South-east Queensland metropolitan area should be used, as it includes areas that people travel from to work in Brisbane's metro area. You could even add northern NSW to the SEQ metro area's population.
The City of London had 9,401 people.
https://en.m.wikipedia.org/wiki/City_of_London
Greater London has 8,961,989
https://en.m.wikipedia.org/wiki/Greater_London
The London metropolitan area includes London and its surrounding commuter zone (the area in which it is practicable to commute to work in London). It is also known as the London commuter belt,[1] or Southeast metropolitan area.[2]
https://en.m.wikipedia.org/wiki/London_metropolitan_area