@Radoye:
Should I decrease the areas ? But perhaps the whole map could be reworked with this respect ? We can see this after releasing the version for betatesters.
Compared to today, a much larger % of the population in what was once Yugoslavia used to live in villages and on farms in the period before WW2. After the war, the Communists collectivized the farms and started industrializing the country to move the peasants away from their land and turn them into city-dwelling proletarians. This process was largely completed sometimes by late 1960's, so all maps from that period and more recent will show urban areas much bigger and built up than they were historically in the period we're interested in.
For example, Novi Pazar town was ~10,000 population before the war, and today is ~60,000, while the total population of the municipality including villages didn't change much.
Novi Sad, which is ~300,000 today was ~60,000 before the war.
And so on, pretty much everywhere, including the biggest cities. Except one, Subotica in the northeast, right on the border with Hungary. It was the largest city in Yugoslavia when it formed in 1918 with ~100,000 population, it had ~100,000 population in 1940, and it has ~100,000 population today!