Nicholson was “bigger” than Keaton. Ditto that Marlon Brando and Gene Hackman were bigger than Christopher Reeve in 1978, the latter a soap opera actor (which was considered the lowest rung on the acting totem pole, below primetime tv actors) who mostly did stage work (and had only a small part in a film the same year “Superman” was released). Reeve wouldn’t have justifiably earned top billing until the second film, even if he was the lead actor.
I’m of the opinion that the protagonist should get top billing, especially if they are the title character. Though it’s been a while since I’ve seen it the first Batman may as well have been called “The Joker” though IIRC.
Wasn’t Brando in Superman in a glorified cameo?