The grip is the only physical contact between the player and the club.  Grip size is determined by:

             1.  Size of a player's hands.
             2.  Player's preference in gripping styles 
                        (overlap, interlock, and ten finger).
             3.  Player’s hand strength

Too small of a grip and the player might hook the ball. Vise-versa, too big of a grip and the player might slice the ball.