During launch there used to be only one datacenter for NA/EU and one for Japan but we ran out of instances (You would queue with a full party but would have a queue time since there were no available instances) so they spun them off into more data centers which is what we have now. Perhaps then Primal had more players per server than Aether did.

It's more a problem of the weird loopholes of getting into (apparently) full servers by server transferring to them making them overpopulated.