I hate to admit this but for a while I have been studying these. There deffinately appears to be a difference in the size of the back of the skull. The jaw size, Rats are bigger and in my experience if you do have a rat skull, Pull out its incisor, If its a rattus norvegicus; You will be greatly surprised that the inscisor reaches right through the jaw and almost through the cheek. Generally speaking the rat is bigger.