Mormon, like his ancestor Nephi, was a large man physically and also was of a spiritual stature that was unique and impressive, especially for a teenager. Perhaps Mormon, like King Saul, was immediately looked to as a leader because of his impressive physical prowess. Undoubtedly the other characteristics of spirituality and maturity, when coupled with his size, increased his stature among his peers.
It could also be that he was a priesthood leader at a young age and as a result people turned to him for leadership in both temporal and spiritual matters. President Joseph Fielding Smith observed:
“We may ... conclude that Mormon received the priesthood at a very tender age. He was only ten years old when Ammaron counseled him and placed in him the wonderful trust as guardian of the sacred plates. Moreover, when he was fifteen years of age he had a visitation by the Lord and ’... tasted and knew of the goodness of Jesus.”
(Answers to Gospel Questions 2:9-10.)
It appears that Mormon was appointed to lead the Nephite armies into battle against the Lamanites not so much because of his physical stature but more likely because he was indeed a remarkable leader in many aspects of his life. We are left only to surmise all of the reasons why Mormon was thrust into weighty responsibilities while so young.
he record does not detail all of the greatness and unique qualifications of Mormon. Mormon the record keeper was also modest and humble.