https://wccftech.com/wow-classic-hotfix-realmpopulation/

Blizzard has released a new WoW Classic hotfix that changes how the realm population labels work.

According to Blizzard, the change was implemented to display population on a particular realm in a more meaningful way. Prior to this hotfix, realm population was calculated by 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”, Blizzard explains.