A mummy bag tapers from the head end to the foot end, reducing its volume and surface area, and improving its overall heat retention properties. It usually does not unzip all the way to the feet because the zipper is a weak point in any sleeping bag's insulating qualities. Together with the tapered shape, this design feature helps protect the feet, which are more vulnerable to heat loss than other parts of the body. See the Wikipedia article on Sleeping Bags.
