The political boundaries of the British Isles have changed over the years, but Great Britain is, and always has been and (barring any Atlantis style submersion or dinasour grade asteroid strike) always will be the largest of the British Isles. That may be pedantic, but, whatever, that's the way it is. It's not a country in iself, it's just the biggest of the British Isles. It's a fairly easy concept to grasp. I can't be bothered to check exactly how many islands make up the British Isles but Great Britain is the biggest.
'The United Kingdom of Great Britain' (to which you refer in your AoU) just has, well, too many words. The words 'Great Britain' are in there, I'll admit, but by default it excludes every other island in the British Isles.