This note was added to the official hotfixes:
Realm populations are now measured against a realm’s absolute capacity.
Developers’ note: 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 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.