Despite Masters only having 4% of players, the range within masters is even bigger than the range in diamond with 20% players. Diamond has a slightly higher range than platinum,gold and silver, which have about the same range.
That's to be expected though, as the maximum mmr probably keeps increasing and only few players can keep up, so the population density gets smaller, the higher the mmr.
That's also a reason why a player staying on the same mmr should be demoted at some point, since the mmr of everyone else (who's active) around him should increase, so they deserve the e.g. master rank more.