First thing to note is just how empty the world is. The population density of the mapped area is a little higher than Alaska, but lower than Wyoming. It is by a large margin less densely populated than medieval France or even the less densely populated medieval England.

One must imagine that most of the population is in or around the marked towns and cities, and that the rest of the cultivated hexes have a village or manor every 10 miles or so with a population of around 500.

Maybe we Just ignore the total national populations, except as relative to one another. We can keep the city and town populations as is.