Mormon compares the Lamanite and Nephite casualty rates—much higher for the Lamanites because they lacked armor. This does not mean that the Nephites were invulnerable. This combat would certainly have been hand to hand, since there would not be space to deploy archers or slingers. The injuries to limbs could easily cause a fatal loss of blood. A battle mural from Cacaxtla, dating from around A.D. 650, shows several defeated warriors, many with wounds on their arms and legs.
Mormon does not mention the relative size of the armies here, although he confirms (v. 51) that the Lamanite army is much larger, even though the Nephites are fighting within their own territory. Had the Lamanite army been smaller, Moroni might well have met them directly, perhaps intimidating them into a second retreat. It is not clear whether the armies would have been matched in size if Moroni had had with him in Manti the detachment left behind to guard Jershon, but Mormon implies that, even so, the Lamanites outnumbered the Nephites. Armament, tactics, and religious motivation, not numerical superiority, made the difference.