Even the studio might not have that control. Credit in Hollywood is tricky business, and putting Colleen on the poster might even violate certain other contracts. It's just literal meaning of "top billing". It's all a weird mess of contracts and guilds. Heck, my name isn't at the bottom despite being on the movie for over twice as long as some of the folks down there, but you won't see me shedding tears about it. It's just the way it is.