They might call a tankobon or a larger volume of One Piece a graphic novel if it was formatted in an episodic way (e.g. the "Romance Dawn" arc, the first 7 chapters, in one book), because at that point it would be very much like serialized graphic novels in the west, or at least the ones that I'm familiar with. Even with the ones that would take a few books, it's not like we don't have things like "John Flodstritum and the Amulet of Nevar, Pt 3" being published as "graphic novels.