The plates that Amaleki gave to King Benjamin were the small plates. The large plates were continually added to and handed down from record keeper to record keeper until A.D. 385, when Mormon received the large plates and abridged them. His abridgement was written on the gold plates or the plates of Mormon.
To help you gain perspective of the number of pages of the Book of Mormon that are devoted to various time periods, see the chart “Nephite Record Keepers” in the appendix (page 408).