The biggest problem with the population of Balmung and Mateus is the activity. Theyre currently sitting at 30k players combined, but 16k of those arent active. Theyre alts and people who rush to make level 1's to have them on balmung in case they want to do things.Both have the highest active player population in Aether in the information I could find, with 19k and 13k. I would suggest that this isn't likely a coincidence.
Conversely, the population of Greg, Cactuar and Jenova add up to about the same number (those are the 3rd, 4th and 6th largest servers in the world) but have ~22k active players vs Balteus's 14k. Thats the ENTIRE POPULATION of Jenova in activity difference.
Balmung and mateus have the worst activity to population ratios in the world, but the metric they used was total pop.