The ranking of city population data is a standard entry among the vast majority of city articles introduction. It is useful to estimate the relative size within a certain framework. The first priority is traditionally the country. Because of Germany´s advanced degree of integration within the EU, this is the second point of reference here. Note that the whole Berlin article profoundly draws data from its EU ties ( Map in infobox, data in Economy section, metropolitan area in intro !) In this respect the EU ranks maintain a certain consistency within the article itself. Because of space 
and relevance restrictions the introduction can´t include endless listings of one city data aspect. The following sentence about the metro area size and rank is the sufficient data to describe the size
of the city population.