Black holes don't have volume. The reason why they exist is that they are so dense that the gravity that they exert is so powerful that it overwhelms any other force that would hold an object to its shape. Objects that we know in our daily lives occupy the amount of space that they do because of electromagnetic forces between molecules. Nuclear forces between protons and neutrons limit the minimum size of atoms.

In a black hole, gravity overpowers everything, and it all collapses to a point of zero size. It does not matter what shape it had originally.