Mormon typically narrates the material he takes from the large plates. When he uses secondary sources, as he does in this case, he will sometimes copy, and other times narrate. We will see both methods as he works through the story of the people of Zeniff in the land of Nephi.
At the beginning we find the declaration that it is “I, Zeniff” who is writing. Mormon is copying the record of Zeniff, and as far as can be known from what he copied, it is the full record that Zeniff created. The much shorter version of this story is found in Omni 1:27-31. We have much more information in this account. While Zeniff notes that he was not the leader of the expedition, he does identify himself as one of perhaps several expedition members who wanted to change the mission from one of destructive revenge to one of reclaiming the lost land. Interestingly, the shortened version in Omni notes that the group wanted to possess the land, which refers to the second expedition rather than the first. As suggested in the comments on Omni 1:27–30, this difference appears to be due to the type of information Amaleki had available to him, which was more hearsay than what we get in Zeniff’s firsthand account.