Do they use the same methods to define "island"?
The section for Australia seems very broad: "Australia itself dominates the islands around its coastal fringe, which range in size from smaller rocks that are not covered by water at high tide to ..."
While it says the US has 18,617 islands, I struggle to find an official source for that very precise number.
I also see how https://en.wikipedia.org/wiki/List_of_islands_of_Florida says "The U.S. state of Florida has a total of 4,510 islands that are ten acres or larger", suggesting that ten acres is the minimum sized used for "island" in the US.
https://en.wikipedia.org/wiki/List_of_islands_of_Maine says "Maine is home to over 4,600 coastal islands, ranging from large landmasses like Mount Desert Island to small islets and ledges exposed above mean high tide."
Clearly these are not using the same definitions.
I managed to find the Global Islands data set at https://www.sciencebase.gov/catalog/item/63bdf25dd34e92aad3c... with an explorer at https://rmgsc.cr.usgs.gov/gie/ which should have exactly what I want, except 1) it only lists ocean islands, not inland ones, and 2) I can't figure out how to get the data by country.
It categories things as "Big Islands (greater than 1 km2), Small Islands (less than or equal to 1 km2 and greater than or equal to 0.0036 km2), and Very Small Islands (less than 0.0036 km2)." "There are 21,818 big islands in the database. The remaining 318,868 islands are all less than 1 km2 and are classed as small islands.'
I give up.