Here's the strange thing though, the territories that dwelt at the base of this list have no more means of sustaining themselves than the ten square kilometres supporting a population of 4,425,720. There's the 56,375 population of Greenland to contend with 0.03 population density but a little problem: zero square kilometres of arable land. A small population reliant on trade with others.
There are fifteen countries in that predicament and they consist of some of the small populations or small dependent states so far surveyed: Monaco, Macau, Gibraltar, Vatican City, Jersey, Nauru, Tuvalu, Tokelau, Anguilla, Norfolk Island, Cocos (Keeling) Islands, Christmas Island, Falkland Islands, Svalbard and Greenland.
Hong Kong has a larger population than Singapore but also a larger patch of arable land - 53 square kilometres. The scale of the disparity between the population and the means within its landmass to be sustainable is too great to quibble about who is most challenged.
Unless you look at the two measures, population density (people per square kilometres) and real population density (people per square kilometre of arable land).
In the first measure, Hong Kong's population density of 6,621 is greater than Singapore's 6,483.
In the second measure, the one with the arable criteria, Singapore has a real population density of 440,998 to Hong Kong's 131,101, switching them quite substantially.
China's performance is a good average of 943 people per square kilometre of arable land.
Bangladesh has fifty countries higher than it on the scale. That's how great the difference is in these measures.