Rodimus was the leader of the Autobots at the end of the Great War, not Optimus. So Optimus must have stepped down for whatever reason (maybe he was off on some idealistic crusade

) and Rodimus became the leader. It was under Rodimus' leadership that the Autobots won the Great War and the Pax Cybertronia treaty was signed. Rodimus also paved the way for the Maximal Upgrade and was one of the first to have himself downsized (indeed the last G1 toy of Hot Rod was a Micromaster!). Rodimus would later become a member of the new Wreckers during the Beast Machines era.