Mg2+ ion is smaller than O2- ion although both have same electronic configuration..why??

coz Mg²⁺ has  lost 2 electrons while o^2- has gained electron

Larger the cation charge smaller the atomic size and larger the anion charge larger the atomic size. Hence on the basis of atomic size,

Mg²⁺<Mg and O^2->O. we know, Mg<O

Hence Mg²⁺<Mg<O<O^2-.

Thus due to increase in nuclear charge, Mg²⁺ <O^2-. 

I mean Mg²⁺<O^2- (In the last line.)

Mg²⁺ and O^2- ions are isoelectronic (they have the same number of electrons). However, magnesium has a greater number of protons than oxygen, and hence greater effective nuclear charge and larger force of attraction with the valence electrons.

More effective nuclear charge