Due to the manner in which FreeBSD gets the memory size from the BIOS, it can only detect 16 bits worth of Kbytes in size (65535 Kbytes = 64MB) (or less... some BIOSes peg the memory size to 16M). If you have more than 64MB, FreeBSD will attempt to detect it; however, the attempt may fail.
To work around this problem, you need to use the kernel option specified below. There is a way to get complete memory information from the BIOS, but we do not have room in the bootblocks to do it. Someday when lack of room in the bootblocks is fixed, we will use the extended BIOS functions to get the full memory information...but for now we are stuck with the kernel option.
Where n is your memory in Kilobytes. For a 128 MB machine, you would want to use 131072.