15 million was a very large population at the time. The France comparison is more impressive than it seems at first-- until well into the early modern era, the French comprised one quarter of the population of Europe! (Read a little further down the wikipedia article you linked, it discusses France's demographic evolution.) The population of Europe had taken a real beating in the Black Death, and it was still recovering when the Ottoman Empire rose to power. Some population estimates. As for the small core population-- while it's true that much of the Empire's population wouldn't have been Turkish, Suleiman's rule was before the rise of the nation-state. Many European countries were ruled by leaders who didn't speak the same language as them. I don't imagine French subjects had much more affection for their nobles than Turkish subjects had for theirs.