since
http://zero-k.info/ladders/games differiates between specs/players, why wouldn't the same work for the pie graphs?
Also is that "players in game" or "players in battle room"? For the pie graphs it is "in battleroom" iirc.
But generally I think zeroK and BA are even when it comes to popularity. For each game there is always 1 large team match going, sometimes it spills over to a second room. Added to that there are some 1v1s and 1-4 people playing chicken or something.
After all what matters most is how at almost any time there is a game going that you can simply join. So after waiting ~45min for a game to start you can play, without having to organize with other players.
Whether that game is 5v5 with 4 specs or 12v12 with 10 specs is just details.