We know that one orbit increases one step at a time from top to bottom of the periodic table. As a result, the more you go from the top to the bottom , the more the amount of electrons increases and the size of the element also increases. The number of electrons in Mg2 + ion is 10 which is arranged in two stages. And the number of electrons in Ba2 + is 8 , which is much more than the number of orbits. Therefore , the size of Mg 2+ ion is relatively small.