Atom
The size of an ion (as measured by its ionic radius) is different than the size of the atom due to the addition or removal of an electron.

Cation
The atom loses an electron when a cation is formed. Since the number of protons in the nucleus has remained the same, there is more positive charge available to attract each of the remaining electrons - the cation is smaller than the original atom.

Anion
The formation of an anion occurs when an atom gains an electron. The number of protons in the nucleus remains the same, but the additional electron means less positive charge is allotted to each electron - the anion is larger than the original atom.