The particles that contribute most to the volume of an atom are electrons. While protons and neutrons reside in the nucleus of the atom, which is a very small region at the center, electrons exist in areas around the nucleus known as electron shells or orbitals.
Even though protons and neutrons are much more massive than electrons, they occupy a comparatively tiny space, making the nucleus very dense and small. In contrast, the electron cloud, where the electrons are found, extends significantly further out from the nucleus. It is this electron cloud that defines the size and volume of the atom.
Because of the nature of electron behavior and distribution, the bulk of the atomic volume is determined by the regions in which electrons are likely to be found. Their wave-like properties allow them to occupy a relatively large space around the nucleus, thereby contributing to the overall size of the atom.