And why 4470 for POS? Did everyone borrow a vendor's FDDI-like default or is there a technical reason? PPP seems able to use 64k packets (as can the frame-based version of GFP, incidentally, POS's likely replacement).
According to this URL http://www.columbia.edu/acis/networks/advanced/jumbo/jumbo.html which you have seen before, the number of CRC bits in the protocol header limits the number of bytes you can practically use for the MTU. I expect that we won't go beyond 9000 byte MTUs for a long time. The 4470 for POS probably comes from Token Ring originally. In the original 4 Mbps token ring a device was allowed to hold the token for 9.1 ms which was enough time to transmit 4550 octets. This timing was probably adopted by FDDI which borrowed a lot from the token ring design. No doubt, the designers of POS were also influenced by token ring and just chose the same size. --Michael Dillon