One of the important aspects of what Chief Captain Moroni did was to cause “that all the people in that quarter of the land should gather themselves together to battle.” Moroni did not bring his entire army, but, in addition to those he did bring, he rallied those who lived in that quarter to come to the defense of the land. Therefore, he mustered more men than just those who came with him.
One of the differences we see in these war chapters from what we have seen before, or will see again, is a more detailed look at armor, armaments, and tactics. As a military man, it is certain that Mormon is interested in such things, but it is also probable that we get the information in these chapters because of his admiration for Chief Captain Moroni, rather than simply because of his interest in military things. Mormon admired him enough to name a son after him. We probably get the details of these wars, not for the wars per se, but as examples of the valor and importance of the men who are prominent in the stories of these wars. These wars are where we learn of Chief Captain Moroni, Lehi, and Teancum. The wars continue into the book of Helaman, and there we will hear of Helaman and his stripling warriors. Mormon is interested in the people involved in these wars, and therefore we get the details that help us understand those heroes.