From the Super Mario Wiki
Mario using Enlarge
Toadette using Enlarge

Enlarge is a special ability in the game, Mario Super Sluggers. Used by Mario, his baby counterpart, and the Toad characters, Enlarge is activated only during close plays. When the player presses the right button first in a close play, the runner grows larger and knocks away the base person. This can also let the ball fly out of the base person's glove and roll somewhere else on the field, which can enable the other runners to advance a base.