It was explicit in their long blue post here:
Why do you find it hard to believe when both classic and retail are run on the retail base client and the same server hardware? Wouldn’t it be weird if they were different despite the client and hardware being the same?
Here is a blue snippet from that thread:
“ 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.”