Well, population has very little to do with it. I mean, if we take the UK, we usually account for 1/3rd of European sales, and yet our population is only around 1/5th of Europe (according to Oyvoyvoyv's 275m number).
I believe it has something to do with the maturity of the gamers. I mean, I remember reading that the average age of a gamer in the UK is 33. By 33 people are usually quite a way up in their career-paths, and often have a family. Put these all together and you'll find that we are going end up spending a lot more money per-person than other countries, whose mind-frames are still in the "games-are-for-teens" mindframe.
Fun-fact: The UK was the second highest spender in Videogames in the world last year (after US) even outpacing Japan whose population is double that of the UK.







