There is significant sound transmitted to the ear canal via the bones around the ear. Muffs are needed to help deaden that sound. It's not a bad idea to use plugs too, but if you're going to use only one method, muffs are probably the way to go..
Exactly, and almost as much inner damage occurs this way - I just got my hearing tested, and one of the causes of my tinnitus over the years had been plugs only with my shotguns along with the occasional hunting shot with no protection. Muffs can be fitted to small heads, even with glasses on.