Wikipedia's Tuque). And for sure, She looks beautiful, and cute as her smile.
Tuque that covered half of her head, became a frame that is useful to show more clearly the features on about half of her face. Especially her chin, jaw, and cheek.
Keira Knightley face shape which is a fusion of a square and a heart, become like transformed into a round. Look at the red path, which curved to follow the edge of her jaw and chin. It looked smooth curvature without breaking, starting from her jaw angle, cheekbones, and chin. And Keira Knightley's face looked so lovely with its Tuque.
Indeed, the face shape may change, where one of the causes is obesity factor on face. Look at her prominent cheek, right on the zygomatic bone, which makes her face look rounded.
Image source: Keira Knightley on Ricky1146's Gallery on Picasa Web Albums.