The datagram sequencer distinguishes between "packets" (bunches of data sent at the same time) and actual datagrams.  One packet may be fragmented into N datagrams.  Datagrams have a hard limit of 1500 (the MTU), but packets need a limit too, since if you lose any fragment of a packet, you lose the whole thing.  So, "maxPacketSize" is an adjustable soft limit.