By curiosity I checked some numbers to compare. Paris density is just crazy! When compared, Berlin (mentioned by cameldrv) really has a low density for the huge capital it is, at around ~3.9k persons/km². London seems to have around 5.5k, New York around 10.2k, while Paris is at a crazy 20.7k!
Numbers from https://versus.com/en/berlin-vs-london-vs-new-york-vs-paris/....