Only in some cases you are ranking the population of the urban areas (for example in the UK, Norway, and Iceland), while for most countries you are actually ranking the most populated municipalities, which include suburbs without any importance.
Moreover, you should accept "Keflavik"; nobody calls it "Keflavik og Nordjarvik".
Citypopulation is an amazing website, but it doesn't rank urban areas for all the countries, simply because there are not enough universally accepted data. So if you really want to rank urban areas you have to do a lot of research.
But what I was trying to point out is that in a game you shouldn't mix methods. For the UK you are ranking urban areas, while for France you're ranking municipalities (that's why Lille is not there), while citypopulation gives amazing data about French urban areas. Same story for Norway and Finland.
Since finding data for urban areas in all countries would take a long time (and for some countries you wouldn't find any) I think that the best solution to make a perfect game would be to just rank the population of city proper for all countries ;)