Major League Baseball is introducing bigger bases this season as part of a flurry of changes designed to put more action and athleticism back in the game. The bases are expanding from 15-inch squares to 18 inches, which cuts down the distance between them by 4 1/2 inches. The distance between third and home and home and first was trimmed by 3 inches. With the new bases, major league teams could try a more aggressive approach on the basepaths. The bases also could help prevent injuries.