Blizzard made a post yesterday in the Community Council forum saying that this is not true. Classic servers and retail servers have the same server cap.
" 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."
Also if you haven’t already read the Blue post from blizzard in that thread from yesterday the TLDR is that they cannot fix the que issue and they suggest that people move servers.