I watched a little of this on CSpan. It looked bigger than 50,000 from their camera angle but from the shot in this thread, I think you are right, about 50,000 bussed in liberals.
That was intentional. The organizers didn't want any other views accept the one they provided. They positioned platforms for camera at a low angle to make the crowd appear larger that it really was. I believe the technique is called "Forced perspective". MSM use that tool all of the time at Obama Speeches.
Rally to Restore Sanity Pledges to Strictly Prohibit Filming at National Mall