It seems kinda weird. In Austrailia, the parts that make up a metro is what most Americans would probably consider neighborhoods of a larger city rather than suburbs as Australians would call them. A lot of it has to do with political divisions it seems.
Los Angeles metro is near 15,000,000 people over 2,500 square miles.
Los Angeles proper is around 4,000,000 over 468 square miles.
Melbourne (metro) is around 4,000,000 over 4,000 square miles.
The City of Melbourne is 93,000 over 14 square miles.
I think it's more of just how the two countries differently define what a city is (and the varying powers of the respective local city governments).