I love this quiz idea but I seriously question the validity of your source. I have lived for most of my life in Sheffield, UK and it has a population of half a million, not one and a half. The implication is that all the other figures are also wrong.
citypopulation.de is like the most reliable source (and it's the most used/accepted one on Jetpunk). It uses Urban-Area, which adds suburbs and nearby smaller towns/cities to the total population count of the city. This makes New York Cities Population for example go from 8 million to 28 million.