@contrex,
Contrex is touching on what I wanted to say, namely that "isle" is often used poetically and descriptively.
I has nothing, as already stated, to do with actual size of the island in question, and should not in my opinion be compared with the word "islet" which by definition is a small island.
Otherwise, it is used as a synonym for "island" and that can run to any size, e.g. Madagascar.