The Island is called Great Britain to distinguish it from Lesser Britain, which is Brittany in France. It's simply a description of size.

It's an anglo norman thing, la Grande Brettayne, because they had borders with Brittany so they needed to distinguish lesser Britain from great Britain. The welsh scholar Geoffrey of Monmouth uses it, for instance.

The first actual instance of it being used is I think the Egyptian scholar Ptolemy, who used Great Britain and lesser Britain to refer to the two islands of the British Isles. (In this example lesser Britain was Ireland). But the modern usage comes from the Normans.