The idea that number of great gniuses is proportional to population is as simple-minded as the idea that national income is proportional to area. Human beings are so complicated, and interact so richly that these simple proportions, which might be OK if applied to fish or rabbits, are completely overwhelmed by cultural complexities.
Western Europe developed science in the early modern period, and that development was intimately braided into european cultural development of the time, religion, government, superstition, wars and revolutions, and the influence of randomly arising individuals all played a part, and we are still seeing that effect today. Area and population just don't amount to a hill of beans causally.