Field of View as has already been mentioned (as focal length) changes the relative size of features due to the relative amount of foreshortening

Also, I think for myself, I tend to make all manner of derpy or weird expressions that cameras capture, made worse by like trying to smile awkwardly and reacting to camera flashes and clicks, whereas in a mirror or even video what is seen is more the more normal collection of those expressions that just looks kind of like a living, moving person.