here is Federer
Notice that his head is quiet, as the eyes are fixed on the contact point before, during and after impact.
Also notice that his body rotates from right to left, around the CENTER of his body, i.e. the spine, which is how a human body is supposed to rotate. (think about how a figure skater rotates)
Here is Roddick
Notice that the head is moving, before, during and after impact. Also notice that his body rotates around the left side (instead of the center). (now the figure skater is looping around his left foot, instead of rotating around the spine)
The difference in these videos explain why Federer's FH is bigger, but with seemingly less effort. By rotating one's body around the spine, one generates more speed towards the ball.
Also, with a quiet head and eyes stay focused on the contact point, his FH is cleaner with better control, less vulnerable to break-downs.
So...... you kids out there, watch and learn!