According to Daniel Ludlow, when Ammaron turned the responsibility of the records over to Mormon, he indicated that Mormon should "engrave on the plates of Nephi all the things that [he] had observed concerning his people" (Mormon 1:4). Thus Mormon's major record of the events of his day was written on the Large Plates of Nephi.
However, later in his life Mormon was commanded by the Lord to make a separate set of plates, the Plates of Mormon (Mormon 3:16). Mormon then abridged onto the Plates of Mormon all of the writings from the Large Plates of Nephi, including his own writings. Concerning his writings on these two sets of plates, Mormon said:
"And upon the plates of Nephi I did make a full account of all the wickedness and abominations; but upon these plates [the Plates of Mormon] I did forbear to make a full account of their wickedness and abominations" (Mormon 2:18) [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, p. 170]