(1) New York - 20.2M
(2) Los Angeles - 13.3M
(3) Chicago - 9.5M
(4) Dallas - 7.2M
(5) Houston - 6.8M
(6) Washington, DC - 6.1M
(7) Philadelphia - 6.1M
(8) Miami - 6.1M
(9) Atlanta - 5.8M
(10) Boston - 4.8M
Only Sydney would make the top 10, coming in just above Boston.
Both the US and Australia have cities / metro areas that are significant in population as well as huge low-density areas, but the US metro areas are definitely larger. This makes sense given an overall population of 323M vs 24M.
For example, the Seattle MSA extends as far north as Burlington, Mt Vernon, as far east as almost Lake Chelan, and as far south as Packwood near White Pass.
No one would credibly claim that White Pass was a part of Seattle, any more than Lake Chelan.
Whereas Melbourne absolutely would consider Boronia on the east a suburb, Campbellfield on the north, and Frankson in the south all suburbs.
I do get what you are saying, and of course with that population number it is so, but I don't think that there's really a great correlation (mainly as a result of Census in the US using a non-standard definition of the internation "metropolitan area" to basically... "anywhere else".
That being said, when you look at "as a percentage of the population", Sydney and Melbourne at ~20%, the others at ~10%, there's also not the argument of "hey, these US cities are far more dense and complex than Australia's", when the complexity is far more often a result of _artificial barriers and restrictions in the market_ than anything logistical.