Here's why:

* Atoms are mostly empty space: Atoms are composed of a tiny, dense nucleus surrounded by a much larger cloud of electrons. The nucleus contains almost all the mass of the atom, but it occupies a very small fraction of its volume.

* Electrons determine size: The size of an atom is primarily determined by the region where the electrons are likely to be found (the electron cloud).

* Copper's structure: Copper, like most metals, has a crystalline structure where atoms are packed tightly together. While the nuclei are close, the electron clouds still contribute significantly to the overall volume.

Therefore, the volume occupied by copper nuclei is a tiny fraction of the total volume of the copper piece.