Major League Baseball is the only one of the major sports that dramatically changes its roster limit rules for the final month of its regular season. After playing five months with 25-man rosters, baseball increases rosters to a maximum of 40 players beginning on Sept. 1, before reducing them back to 25 for the playoffs and World Series. The only reason this rule has ever made sense is that the minor league teams that provide the inventory of extra bodies stop playing in early September.
However, this roster change causes imbalances. Large-market clubs usually have the freedom of calling up any player they want while small- to middle-market teams usually are limited to the number of players they can call up, a number they have most likely budgeted in the offseason, regardless of whether they are in a pennant race. The other imbalance is the level of talent that each club has at the Triple-A or Double-A levels, or the specialty players it has available to help in a tight pennant race.