The actress's chest was airbrushed in the promotional image for the 2004 action-adventure film.
Keira, 27, told Allure magazine: "They always pencil in my boobs. I was only angry when they were really, really droopy.
"For King Arthur, for a poster, they gave me these strange droopy tits. A - I don't have tits anyway, and B - they digitally made them, and I thought, Whoaaaaa! It's my face on that poster.
"I thought 'Well, if you're going to make me fantasy breasts, at least make perky breasts'."
She told the magazine: "Normally the woman has to expose everything and the guy doesn't. Sex scenes in films - I'm quite rigorous about what gets exposed. No bottom half!
"I don't mind exposing my tits because they're so small - people really aren't that interested. It's much easier as you get older. You can say, 'No', 'Yes', 'No'."
Keira, who was photographed topless by Mario Testino for the magazine, recently got engaged to James Righton, the Klaxons keyboard player.
The Pirates Of The Caribbean actress said that she had no idea how she will handle the publicity when they decide to wed, adding: "Maybe I'll be permanently engaged." She said that she and her fiance are "total opposites" because she "knows nothing about" music.
:: Allure is available internationally through the iTunes store.