The term 'Great' just means 'largest' - it being the largest island in the British Isles. As far as I'm aware, the 'Great' in Great Britain refers to the 'largest' island of the British Isles.
OK, so I have been wondering. Lets assume that I am a young man visiting London and I meet a pretty girl and we get along. One day she asks me if her butt looks big in her jeans and I reply... "you have a great arse". Did I just blow it? Did I answer, "yes, you have a large butt"?
Inquiring minds wnat to know.
J