This reveals something many readers might not have noticed. It's a quirk of politics and history that "Sydney" is defined to include the entire urban agglomeration that surrounds its city core (all 4750 sq miles of it), but this isn't the case in most U.S. cities. For an apples-to-apples comparison, you should be looking at U.S. CSA (Combined Statistical Area) populations (not the populations of core cities).

That puts Sydney outside the top 10 U.S. metros -- considerably smaller than #11 Atlanta (6,555,956), and about the size of #12 Detroit (5,336,286) or #13 Seattle (4,764,736).

The Los Angeles CSA (since you brought it up) has ~18 million people in it. †

† In the spirit of fairness, it should be noted that CSAs sometimes cover extremely large areas. If we were to restrict Los Angeles to its MSA (which at 4,850 sq miles covers an area almost identical in size to Sydney), its population drops to about 13 million. The difference between CSA and MSA populations isn't usually so large, but the Los Angeles metro area contains an almost ridiculous amount of urban-ish sprawl, compared to most other cities.