To be fair, Ethan's sword also wasn't 3 1/2 Zoros tall before, either. Somehow, he's apparently able to change the size of his sword, to match his Bakotsu hybrid scale. He may also be able to change how it looks to suit his needs as well? Getting rid of the guard to make it more "aerodynamic" and draw faster, maybe?
Maybe that's part of his "Yokai" powers? Maybe that's a thing Black Blades can just do that they've never mentioned before?
I have seen it mentioned that, given Oden's size, Zoro shouldn't be able to wield one of his swords, given that it would just be too long.
Maybe...scale actually matters in One Piece? A crazy thought, I know. Probably shouldn't even say something so outlandish.
But, it is getting me thinking. (Always dangerous, I know). If Black Blades CAN alter their size and form to suit their wielder...could it be they aren't actually "swords", at all? Is there a different way they're meant to be used? They're being used as swords, but are you supposed to use them in a different way? Some different function that they're meant to be used for, after they become Black Blades? I'm not sure what, but...I don't know, I'm still thinking about it.