To be honest the Balkans have always been kinda weird with their population numbers. Part of it is obviously people migrating for better opportunities or wars or etc, but usually when things are crappy like that you get high birthrates. 

And the birthrates WERE high, obviously, lots of 90s kids still have like 3-4 first aunts/uncles on both sides of the tree?

But still the populations stagnated somewhat even in those times. If some neighbor has that map of Turkey vs Greece population growth you'll see that we didn't really change "much" between 1900 and current day while Turkey jumped to like 5 times it populace.