At left: the British poster/cover image for King Arthur.
At right: the US poster.
According to this article, American marketers don't think Keira Knightley's chest is big enough to attract American consumers. Knightley adds that American magazines won't let a woman appear on their covers unless she has at least a C-cup, or agrees to digital enhancement. The frosting on the cake is that some readers who commented on the article urge Knightley to get bigger breasts.
Notice also the lip enhancement, and the fact that they pared down Knightley's already-wafer-thin waist. And it looks like they were squeamish about the shadows in her armpits, too. How unseemly!
Hopefully I'll manage to post a sketch or two of the young lady later today, as part of a Drawing Board exercise.