electrons, due to their smaller mass and thus larger space-filling properties as matter waves, determine the size of atoms and molecules that possess any electrons at all. thus, anions (negatively charged ions) are larger than the parent molecule or atom, as the excess electron(s) repel each other and add to the physical size of the ion, because its size is determined by its electron cloud. cations are smaller than the corresponding parent atom or molecule due to the smaller size of the electron cloud. one particular cation (that of hydrogen) contains no electrons, and thus consists of a single proton - "much smaller" than the parent hydrogen atom.