Those percentages all seem way lower than I expected. Looking at the metropolitan areas, things change drastically. Milan, for instance, is larger than Rome. Manchester and the Birmingham region are a third and a quarter the size of greater London respectively. Barcelona is only slightly smaller than Madrid, while Munich is over half the size of Berlin.

Also, the size of cities would seem to lend itself to virtuous or vicious cycles- if something is small, there isn't as much to see and do and more importantly, enough economic activity around. Therefore tourists and big companies aren't interested in going there. And so there is no money to build more stuff that would attract people, and there aren't enough people to generate interesting diversity.