Extreme height restrictions combined with extreme regulatory costs is what has lead to this issue.
Show a European politician, especially a local one in charge of urban development, an image from Tokyo and they will recoil in horror.
Here in Europe everything must be flat and look cultural.
https://en.wikipedia.org/wiki/List_of_European_Union_cities_...
As you see, Spain has 12 cities among the top 38 cities in Europe. L'Hospitalet (an urban centre close to Barcelona) is densest than Paris.
Spain seems to have planning laws that force density. Small agricultural towns in the middle of nowhere have people living cheek by jowl in apartments and townhouses, with an abrupt cutoff once you hit the town boundary.
You could weigh the density by population (effectively giving you population²/area?! I'm not saying this is a good idea), and you'd get a top10 of Paris, Barcelona, Madrid, Bucharest, Berlin, Athens, Milan, Brussels, Vienna, Naples, which despite the slightly bizarre metric seems a more sensible ranking (Emperador is at the bottom rank), and which, to be fair, also features two Spanish cities.
But again, it's kind of a pointless endeavor, because of the arbitrary nature of the boundaries chosen -- why Paris and not Paris metro? etc. I guess ideally you'd have a function density(person) giving you the population density of any given person and you'd want to look at the distribution of that function, specifically the average per country of that function.
German towns and cities feel a bit denser than the UK (I live in Switzerland and visit Germany every month).