Why are cations smaller than the atoms from which they are formed?
Neutral atoms that gain or lose an electron are classified as anions or cations. Cations are neutral atoms that lose an electron, becoming positively charged. Ionization represents the process by which an atom or a molecule gains or loses an electron, by means of collisions between subatomic particles.
The size of cations decreases with respect to the initial size of neutral atom, since the force of remained protons increases, after the atom loses an electron, they being able to draw the remained electrons toward the nucleus. The ionic radius of the cation is reduced at each further electron losing.
For example, if Fe^(2+) cation loses one more electron, becoming Fe^(3+), the ionic radius of 76 pm Fe^(2+) decreases to 65 pm at Fe^(3+).