According to the Book of the River Nations: Exploration and Kingdom Building each hex is approximately 125 square miles in area. Each hex, if claimed by a kingdom, contains about 250 people. Thus the kingdoms in the Kingmaker rules have a population density of about 2/square mile. This might be appropriate for a burgeoning kingdom, but what about more established ones?
The average population density for a fully-developed medieval country is from 30 per square mile (for countries with lots of rocks, lots of rain, and lots of ice—or a slave-driving Mad King) to a limit of about 120 per square mile, for a land with rich soil, favorable seasons and maybe a touch of magical help. No land is wasted if it can be settled and farmed. There are many factors that determine the population density of a land, but none as important as arability and climate. If food will grow, so will peasants. If desired, exact density can be rolled randomly, and land arability reverse-engineered from the result. A roll of 6d4x5 will do the trick nicely. Reduce the x5 multiple by any amount down to x1 to represent a much less developed land, or to represent countries depopulated by invasions, plagues or other calamities. Nations hit by such troubles can stay depopulated for centuries, too, barring an influx of immigrants: natural population growth is usually glacial in pre-industrial worlds.
Some Historical Comparisons: Medieval France tops the list, with a 14th-century density upwards of 100 people/sq. mile. The French were blessed with an abundance of arable countryside, waiting to be farmed. Modern France has more than twice this many people. Germany, with a slightly less perfect climate and a lower percentage of arable land, averaged more like 90 people/sq. mile. Italy was similar (lots of hills and rocky areas). The British Isles were the least populous, with a little more than 40 people per square mile, most of them clustered in the southern half of the isles.
So, while a newly established kingdom is likely to remain quite small for some time, what if the players are, for instance, taking over a kingdom that's been around for a while? Does it really matter for the rules, if there are 12,500 people in a hex as opposed to 250? What changes, if any, should be made to reflect higher population densities?