Although Mormon compares the number of prisoners only to those slain on both sides, it seems obvious that the Lamanites still outnumbered the Nephites. They could have continued the fighting and killed a significant number of Nephite soldiers, but they surrendered because they had lost their leader and were ultimately doomed—a core of confused but unengaged soldiers with fighting going on all around them on the periphery. If breaking through was not an option (and it would have taken an organized effort which they were apparently unable to supply, given the lack of leadership), then the Nephites would have worked their way steadily inward. Surrender was the more logical option.
Text: The 1830 edition does not end a chapter here.