Change to navy, because you have black medium which is smaller so you would have something smaller in navy as well. Thats what I would do anyways.
Dbaglover, thanks for your advise. Agreed to your point, for the smaller bag, I've classic medium already, but I would bring it out for a special event only! So, I bought the black one, cuz it is a multi colour, but the navy blue make me can't sleep!