Keyra Knightley has small breasts but not every time as she sometimes displays larger boobs and a more generous cleavage than she usually does, thus casting doubt regarding her real bosom size. Well, it is a widely known fact that she is flat chested and that she has described her chic mammary adornments as "pecs" but still, in some flicks, she sports a surprisingly well-rounded rack. If it's digital enhancing that you're thinking about, it's hardly the case as Keira's breasts have never been subjected to such modern means of augmentation, but rather to the more simplistic yet outdated art of make-up.
In other words, Keira had her breasts skilfully painted on and she has recently revealed such facts in an interview about enhancements she had to go through for her role in "Pirates of the Caribbean" in which you may recall her lumpy corset-tightened bust. It was all an illusion made possible by the old film-studio art of "bosom make-up" which is "a skill from Hollywood's golden age that fell out of favor with the advent of silicone enhancement". Keira declared that she enjoyed the whole process especially because of its age and efficiency: "They painted my t**s on me for the films, which is extraordinary because it's kind of a dying art form - in the past, they used to have whole sections of the studios devoted to bosom make-up." while adding I "loved it, completely loved it. Because it was the first time in my life I had big t**s, and I didn't even need surgery."
It seems to me that whoever was in charge of her make-up did a hell of a good job. In case you're wondering how such a real effect was obtained, a make-up artist would apply a slightly darker shade of base make-up between Keira's breasts to create a shadow to increase the "cleavage effect" in a session that would take up to 45 minutes each morning before filming. Personally, I couldn't agree more with such methods that are unfortunately slowly dying with the ever-increasing use of CGI and other reality-altering digital means.