Actually, he's much more then that. He's basically the Galactus of the transformers universe. He is planet sized Robot searching for other worlds. He can transform into a large planet, and is basically the god of chaos in this verse and can devour entire realities.
This movie was incredibly disappointing for me. Not nearly enough scenes with Quintessa and evil Optimus, so him turning evil and turning back to being good held no weight. The overall plot of the movie went WAY too slowly, they dragged out a plotline that didn't need to be nearly that long (the whole spear and Witwicky thing). There weren't any good fight scenes at all IMO (Bumblebee vs Optimus was ok, but tbh Bumblebee shouldn't have been able to fight Optimus that well given the power levels established from previous movies).
Overall, the most interesting part of the movie was the Quintessa scenes, and she was only on screen for like 2 min. That was the whole purpose for this movie existing, it was supposed to be the big plot driver, and it was relegated almost completely to the sidelines until the underwhelming finale.