I'm reading The Mists of Avalon, and the author (Marion Zimmer Bradley) keeps mentioning "Lesser Brittain." And that got me thinking: if there's a Great Brittain, wouldn't there be a Less/Lesser Brittain? I've simply been assuming that it was called only "Brittain" originally (the London area), and that the other kingdoms, i.e. Wales, Scottland and Ireland, together with Brittain made Great Brittain. Does this make sense? Anyway, I'm just wondering what Lesser Brittain was, if it ever did exist.
EDIT: I'm very proud of myself for consistently spelling Britain wrong. SMH
I haven't read The Mists of Avalon, but Lesser Britain is a name for Brittany, one frequently used in Arthurian literature. This partly comes from the fact that Geoffrey of Monmouth, arguably the pivotal figure in Arthurian literature, called Brittany Britannia minor. The Middle English equivalent was also used, though it often didn't need to be. 'Britain' was not commonly used to mean the island of Britain in Middle English, however, because Britain was not culturally or politically unified. Welsh writers sometimes used the name 'Prydain' for the island but more often it meant areas peopled by speakers of Brythonic languages.
On the topic of speakers of Brythonic languages, Brittany is called as such because its people (possibly just its elite in earlier years), spoke a Brythonic language. The fifth and sixth centuries saw migration from Great Britain to Brittany. Secular and ecclesiastical elites established themselves and their British identity in the region that had been called Armorica. By the sixth century Frankish sources were calling the region 'Britannia'; rather like how Gaul under Frankish elites became known as 'Francia'. When the later French came to talk of the island of Britain, they called it 'Grande-Bretagne'; literally 'big Brittany'. England's Norman elite (which actually included a few Bretons) came to refer to the island as such, and continued Francophone influence ensured the name stuck.
How much contact there was between Britain and Brittany in the 'Arthurian' period is unclear, thanks to an almost complete lack of written sources and a relative lack of archaeological evidence. A British king called Riothamus took an army to western Gaul in around 470, but we can only guess at his relationship with the Bretons. Gildas refers to Britons fleeing over the sea to escape the Saxons, but the accuracy of this statement is uncertain. Breton hagiographies, in particular the Vita Sancti Samsonis, are the most detailed sources, but they were written centuries later.
Sources The Life of St Samson of Dol, The History of the Kings of Britain, The British Settlement of Brittany