I'm no fan of the current division alignment in major league baseball. Each league has a different number of teams, and there are three sizes of divisions. When MLB went to the current three-division alignment in 1994, each league had 14 teams arranged in divisions, one with four teams and two with five. With the 1998 expansion, each league added a team and the Brewers moved from the AL to the NL. The AL now has 14 teams and the NL 16. The AL has division sizes of 4, 5 and 5, while the NL has sizes of 5, 6 and 5.
This poses a bit of a problem for the post season picture. It is easier to win a division with fewer teams in that division, in terms of straight odds. And it is also easier to finish second in that division and have a shot at the wildcard. Now for some geeky math. The odds of a team winning its division are 1/x, where x is the number of teams in that division. That's obvious. But the odds of finishing in second place is also 1/x. Since there are three divisions, the odds of making the playoffs by finishing second (wildcard) are 1/3 of 1/x, or 1/3x. The odds of a team making the playoffs are the odds of winning the division plus the odds of winning the wildcard. So, that would equal 1/x + 1/3x, or 3/3x + 1/3x = 4/3x, with x being the number of teams in that division. For teams in a four-team division, the odds are 4/3(4) = 1/3, or 0.333. For teams in a five-team division they are 4/3(5) = 4/15, or 0.267. For teams in a six-team division they are 4/3(6) = 4/18, or 2/9, or 0.222.
Making the playoffs gives each team (roughly) the same odds of winning the World Series. So, for teams in a four-team division, the simple random odds of winning the WS are 50% greater than those teams in a six-team division. Is this what baseball wants? I think it better to balance the number of teams in each division, even if it means screwing up interleague play, than to create artificial favorites.