Quote - Idyllic feminine faces generally tend to look younger (see neoteny) than idyllic masculine faces; they are generally smoother and rounder, more flesh and fat than muscle and bone.

 

For a good example, try to find pictures of female bodybuilders or physique competitors (ignore everything else, just pay attention to their faces) during a competition and off-season. With a few exceptions, during a competition, when they've pulled their body fat down to minimum levels and have little fluid their faces are typically described as "mannish" because they appear more angular, whereas the same woman off-season, when they have the same amnount of muscle but it's smoothed over with more normal levels of fat and fluid, can look much more stereotypically feminine. Obviously there's zero difference in bone structure, and the muscles in the face aren't any different.