What is the primary reason that cations are smaller than their parent atoms?

Cations are smaller than their parent atoms primarily because of the loss of electrons. When an atom loses one or more electrons to form a cation, the overall size of the electron cloud decreases. This reduction in the number of electrons leads to a greater effective nuclear charge acting on the remaining electrons. As the attraction between the positively charged nucleus and the fewer surrounding electrons increases, the electrons are drawn closer to the nucleus, resulting in a smaller ionic radius compared to the neutral atom from which it originated.

This process significantly reduces the electron-electron repulsion that is present in neutral atoms, effectively allowing the remaining electrons to occupy orbits that are much closer to the nucleus. Consequently, the cation's size is diminished relative to its parent atom.