The Anna Karenina actress recalled an edited version of the King Arthur poster giving her bigger breasts, which she described as "saggy".
"I was only angry when they were really, really droopy," she told Allure.
"For King Arthur, for a poster, they gave me these really strange droopy tits. A... I don't have tits anyway and B... they digitally made them, and I thought, 'Whoaaa!'"
She added: "It's my face on that poster. I thought, 'Well, if you're going to make me fantasy breasts, at least make perky breasts'."
Knightley also stated that she doesn't mind exposing her breasts for a movie role, but will not go full frontal.
She said: "No bottom half! I don't mind exposing my tits because they're so small - people really aren't that interested!
"I think it's much easier as you get older. You can say, 'No'. 'Yes'. 'No'."
Keira Knightley was recently named in a list of 'Britain's Most Naturally Beautiful Women', which was topped by Kate Middleton.
Last month, the actress said that she is not "prudish" when it comes to nudity.
Watch a trailer for Anna Karenina: