So this isn’t exact on how many more people are packed into realms. Before they broke the ability of census addons to report more accurate data it looked like medium realms were roughly twice the size of the average full realms. This is backed up by a quote from Kaivax during the very first initial days of Classic.
Later we can see that they also altered the queue logic to allow an even greater amount of people to be on at the same time.
Later they updated the logic so that it displayed the actual status of the realm, so that we weren’t seeing medium size realms with login queues, which is obviously ridiculous.
So, all indications point to our realms definitely being larger than the vanilla realms were. We have evidence that this is the case based on CM communications posted above, and we have no CM communications to the contrary since. Therefore we have no reason to believe this has changed since.