Look at the size of the country (around 1/3 of USA) and the number of people living there (112M according to wikipedia), also 1/3 of USA. So the density should be about the same but when you look at satellite photos it's one giant city (18M), several smaller cities and the endless forest. Can it support other 90M people?
Also keep in mind the US is very sparsely populated after all. You can easily drive hours in parts of the western US (never mind the parts you cannot even drive through, or Alaska) without encountering a single human settlement.