The idea of Optimus being rebuilt from a smaller, weaker robot originated in the original G1 episode "War Dawn", a fan favorite. It is actually one of the few episodes I remember watching from my childhood. In the episode the Aerialbots travel back in time 9 million years before the great war to discover a lowly dock worker named Orion and his friends, Ariel and Dion. Soon enough "new" flying Transformers show up that we already know are the Decepticons, but Orion is tricked into helping them and in the process is fatally wounded by Megatron. He is recovered by the Aerialbots and taken to Alpha Trion who rebuilds him into the future Autobot leader. The whole point of the episode was to teach the Aerialbots just how bad of a guy Megatron was, but in the process it ended up creating a very interesting backstory for Optimus. Subsequent media has altered Orion's original function from dock worker to Archivist and have pretty much removed the physical differences between Orion and Optimus, except for maybe a few minor details. But originally the character was a completely different robot from Optimus and unfortunately he was one of those obscure guys that we never received in toy form.
|Orion's original Animation model (with Ariel)|
|Orion and Dion|
|The stickers were especially meant to echo Prime's own stickers|
|It really isn't that much of a stretch to imagine it.|