Alma mentions two sets of records: the plates of Nephi (v. 37) and the brass plates. (Of course, Helaman will write upon the first, not on the brass plates.) Modern readers of the Book of Mormon understand that there were four sets of plates, the brass plates, the large plates, the small plates and the plates of Ether. The plates of Ether are not mentioned now because they are not part of the Nephite record; they are explicitly mentioned in v. 21. There is no textual distinction between the small plates of Nephi and the large plates. In the text, they are simply “the plates of Nephi:” “And now, as I have spoken concerning these plates, behold they are not the plates upon which I make a full account of the history of my people; for the plates upon which I make a full account of my people I have given the name of Nephi; wherefore, they are called the plates of Nephi, after mine own name; and these plates also are called the plates of Nephi” (1 Ne. 9:2).
However, I think that more than simple nomenclature is involved here. When Mormon inserted the small plates of Nephi bodily in his record, he commented: “After I had made an abridgment from the plates of Nephi, down to the reign of this king Benjamin, of whom Amaleki spake, I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi” (W of M 1:3).
Mormon implies that the small plates were among numerous records in a storage place and that he did not find them until he had reached Benjamin’s history. During Benjamin’s reign, the small plates were entrusted to the guardianship of the large-plate holder. Possibly the small plates were included in the records of Benjamin’s reign and were otherwise no longer separated from the large plates. As I noted in the original discussion of Nephi’s plates (Behind the Text, Chapter 3, “Mormon’s Sources”), it seems probable that sheets for both records were hammered out to the same physical size and that inserting small plates (small meaning “fewer sheets,” not measuring smaller dimensions) would not stand out in the source materials Mormon was going over.
Reference: Verse 5 references a Nephite tradition about the brass plates: “if they are kept they must retain their brightness.” 1 Nephi 5:19 contains the original prophecy: “Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time. And he prophesied many things concerning his seed.” (See commentary accompanying 1 Nephi 5:19.)