Radak: It is the greatest of the giants, like it says (14:15) [that Arba was] "ha'Adam ha'Gadol ba'Anakim." We find that Avi refer to greatness - "Avi Kiryas Ye'arim" (Divrei ha'Yamim 1 2:50), and [Na'aman's servants called him] "Avi" (Melachim 2 5:13).