Compare a sodium ion and neutral sodium atom in size. what causes the size difference

A cation is smaller than its parent atom.

Therefore, here a sodium ion is smaller than the neutral sodium atom.

Reason: Since the sodium atom loses an electron to form an electropositive ion (cation), its nuclear charge increases (the number of protons exceeds the number of electrons) and the orbital electrons are pulled with a greater energy.

Thus, the sodium ion shrinks in size.