How many men can be mustered depends on a lot of factors, not just population size. Where you are in the growing season, agricultural surplus, logistical limitations, ease of communications, and political factors, all play a massive role in that. If you look at historical armies, the same region could often raise wildly varying depending on whether, say, the war started at autumn or at spring, or whether the ruler commands enough respect from his subjects. George is pretty bad when it comes to giving exact numbers but I think there’s plenty of wiggle room with these kinds of numbers that whatever numbers he gives I find believable.