Words of Mormon
Mormon apparently did the abridging late in his career as leader of the Nephite armies and as keeper of the sacred record. I suspect that he did most or all of it during the four years at Cumorah while they were gathering all the strength that they could muster before facing the Lamanites for the last time. This also explains why we received words such as those of Omni and his sons which had little value beyond giving a date or two. This was the unabridged record. Had Mormon abridged the record those words would have been removed and possibly others. Instead he felt that this record was pure enough that it would be wiser to leave it intact.