Just popped by to comment that truth to be told, I think Marie may be not only a multiple form weapon, but also a nod to Okubo REALLY doing his homework. You see, the original Mjolnir, Thor's hammer, was really damn tiny. It could grow larger, but even when in full size it still remained pretty small as far as hammers go, the handle was just big enough to fit in Thor's hand and the head was not much bigger either.

Of course, if he had done his homework Stein would also be able to throw Marie and have her home in on Justin and then return to Stein's hands... but who knows, he might be saving that trick for later on.