The size of an atom is primarily determined by the distribution of its electrons, rather than the mass of its nucleus. Electrons are much lighter than atomic nuclei, and they occupy the majority of the atomic volume.
In an atom, electrons exist in various energy levels or orbitals around the nucleus. These orbitals can be thought of as regions where the electrons are most likely to be found. The size of an atom is often described in terms of its atomic radius, which is the distance from the nucleus to the outermost electron shell.
While the number of protons and neutrons in the nucleus does contribute to the mass of the atom, it doesn't significantly affect the size of the atom. For example, the radius of an atom of a heavy element like uranium is not significantly larger than that of a light element like hydrogen.
It's worth noting that there are exceptions to this general trend. In certain cases, heavy atoms with larger nuclei can exhibit slight expansions in their atomic radii due to the increased electron-electron repulsion within the electron cloud. However, these effects are relatively small compared to the dominant influence of electron distribution in determining atomic size.