Well, If It Is an island of Wano, It Is by definition smaller than Wano itself. If not, that (Onigashima) would have been the landmass. So, Marco could have just said "that island" .
But to be honest, what makes the deal for me Is this (and some other pics similar to this one):
We saw that Onigahsima was not even big as the flower Capital, that as big as It might be surely puts Onigahsima in the small Island spectrum.