From the blue post:
“The issue we have here is also an issue on certain modern realms such as Illidan and Tichondrius when new expansions are released. They have the same realm caps we have. This is purely a perception thing, we are using the same hardware, and backend infrastructure for both games.”
You can read the whole thread here:
Everything being cross realm in retail is a big reason that mega realms don’t need to exist. There is no fear of not being able to do content because you can play with anyone at any time.