Matter is made from atoms and they are largely empty space. The volume occupied by the nucleus is tiny in comparison to that of the atom.
Very large forces, such as those on the surface of a neutron star or near a Black Hole, could crush a large volume of water to a very small volume, say swimming pool volume to matchbox volume.
This would leave essentially a super-dense volume of neutrons. If the external crushing force was even greater, the "neutron degeneracy pressure" would be unable to resist, and it would shrink out of existence, occupying zero volume.