Where is great in "Great Britain" coming from?

Solution 1:

Ptolemy referred to the larger island as great Britain (megale Britannia) and to Ireland as little Britain (mikra Brettania) in his work, Almagest (147–148 AD).

Later on it was called Brittanie Majore to distinguish it from Brittany in France, which was also mostly inhabited by Celts.

Solution 2:

Great Britain

c.1400, Grete Britaigne. As opposed to Brittany.

Brittany

French Bretagne, named for 5c. Romano-Celtic refugees from the Anglo-Saxon invasion of Britain who crossed the channel and settled there (see Britain). The Little Britain or Less Britain (lasse brutaine, c.1300) of old, contrasted with the Great Britain.