It's not only in the head, if you take a close look! :) http://pic90.picturetrail.com/VOL236.../405997278.jpg
Note how the horse in the first set of images is rather strung/stretched out. The back is hollow, and if you take a look at the trot strides-- the length between the back legs is shorter than the length between the front, and he isn't reaching very far under himself, indicating that this horse isn't using his hind end as efficiently as he could be. http://pic90.picturetrail.com/VOL236.../405997156.jpg
In the second set of pictures, the horse does have a pretty "head set," but he is accepting the bit contact and doing much more! Look at how engaged his hindquarters are-- how far he is reaching under himself to meet with that front hoof! There isn't a gap between his front and back stride. His hind end looks rounder and his back is less hollow.
A horse like this is strong. I think what churumbeque is trying to say is that strength and proper movement are part of balance, and that a horse with this kind of form will balance much easier than one that is strung out!