Great Britain is called "Great" to distinguish it from Brittany in France.
The term "Great Britain" arose in the medieval period to differentiate the larger island of Britain (Britannia Maior or Greater Britain) from Brittany in northwestern France (Britannia Minor or Lesser Britain). This distinction became common practice around the 12th century. The "Great" signified the larger of the two regions historically inhabited by Britons. The distinction didn't come from classical authors; it was a later medieval development.
