The size of the player base is unknown – SEMC guards that information very closely. There are some clues that the ACTIVE player base is MUCH smaller than most of us think it is, however:
Although it’s common for high tier players to face the same opponents on a frequent basis, given that there aren’t as many players in those ranks, I’m in the mid tiers – which are the most populous – and even so, I see the same players over and over in my matches. That almost never happened previously, but over the last 3 months, it happens on a regular basis.
Along with other information I’ve seen, it leads me to believe that the size of the player base is the major factor in the poor matchmaking these days.