With a hotfix to WoW Classic a few moments ago, we’ve changed how the realm population labels work in order to display realm populations in a more meaningful way.
How it worked before this hotfix
As we explained here, the system previously calculated the population labels by comparing all realms to the most-populated realm. This could result in a very crowded realm (with a lengthy queue) showing up as “Medium”, since it was being compared to the realm that had the highest population and a potentially massive queue.
How population labels work now
This system now measures each realm against a realm’s absolute capacity. Medium indicates a healthy total population on a realm – well in excess of what we considered Full in a 2006 – with room for more players to log-in. You’ll now see a High label if the realm is close to having a queue, and a Full label if the realm currently has a queue.
We will continue to monitor this system closely and take further steps if necessary.
Thank you very much!
Source: Official Post