So, Opera's answer is almost right, but the difference between it and the correct answer is too long for comments.
Master league is believed to be the top 2% of all active players in a region, and is believed to cut away the top 10% of Diamond league. This would put the approximate league populations at 2%/18%/20%/20%/20%/20% for active players. An active player is defined as a player who has played a certain minimum number of games per week. Going inactive does not demote you, you just no longer take up a spot in your league's percentage of the active population. The percentages are also not hard values and there is some slush involved to allow them to even out over time.
Quote from Starcraft 2 Forum's MVP ExcaliburZ
So what does Active really mean?
It's technically not a hard 2%. It's kind of complicated and our understanding of it isn't 100% thorough yet because we just learned a lot of this on Friday and there are still many questions that remain. I'll try to explain.
First, it's 2% of active players. That means that if you are within the top 2% of other players who are considered active, you are eligible to be promoted.
This raises the obvious question of "what does the system consider to be active?" Active status hinges upon your bonus pool, since the less active you are, the less bonus pool you are spending. You get 1 point of bonus pool every 112 minutes, or 90 per week, meaning 90 bonus pool is equivalent to one "bonus week." From what we understand there are several tiers of activity.
- Active players -- Players who spend 90 or more bonus pool per week and have less than X unspent bonus pool.
- Semi-active players -- Players who have less than X unspent bonus pool.
- Inactive players -- Players who have X or more unspent bonus pool.
X is something that we're still trying to figure out. It could be a flat value, say 6 bonus weeks' worth, or it could be a percentage of total bonus pool accrued throughout the season. We're also unsure of whether tiers 1 and 2 are the same.
Now, back to the league populations. If you are a semi-active or inactive player and you are matched against another semi-active or inactive player, the system does not gain very much information from that because there is a lot of uncertainty surrounding each player's MMR. It gets the most information by matching you against active players. Note that the system doesn't prefer active players in searches, it just so happens that you're more likely to be matched against an active player because they play more often. If you beat the active Master-level players (which compose roughly 2% of the active population) enough that your moving average moves comfortably into the Master region, you can be promoted.
Second, it's important to know that promotion and demotion also hinges upon a certain confidence level. If you start in Bronze, for example, in order to get promoted into Silver you would have to raise that moving average a fair amount of the way into Silver to prove that you belong there. Demotion is no different.
Therefore, it doesn't inherently mean that by getting promoted that you automatically boot someone else back down to Diamond. For example, if one of the people already in Master league goes inactive, then nobody will get demoted. If, on the other hand, all the active players are still playing games but someone's moving average has fallen into around the mid-Diamond region (thereby proving to the system that that person didn't actually belong in Master league), that person would get demoted.
Hope that answers your question, I understand if that's somewhat confusing. Let me know if you have questions on it and I'll try to clarify further. =]
So what does this mean for your original question?
While technically all leagues should have equal distribution (except for Diamond and Master), in reality, we'll see a much larger number of users in lower leagues as they have been dubbed inactive. If we look at the distribution of bonus points across leagues, its not hard to see that players in lower leagues have many more bonus points, and thus are more likely to be dubbed inactive, and not taken into account for the distribution.
I guess the ultimate TL;DR here is:
Its even for active players, and lower leagues have more inactive players, thus more players.