Cations are positively charged ions.They are formed when an atom lose an electron during a chemical reaction.
Na --> Na+ + 1e-
Na is larger than Na+. This phenomena happens because when the atom has more protons than electrons (for cation), the nucleus tends to attract the electrons inside the atoms thus decreasing the overall radius of the ion. If there are equal number of electrons and protons, there is no changes in the radius. Also, since the ionization tends to lose the valence (outer) electrons, the furthest electron now is being reduced thus accounting to the decreased distance of the atom.