The atomic and ionic radii of group 13 elements are smaller as compared to corresponding elements of group 2.In other words, effective nuclear charge increases and thus, size decreases.Therefore, the elements of this group have smaller size than the corresponding elements of 2nd group.As we move from left to right in the period, the magnitude of nuclear charge increases but the electrons are added to the same shell.Since the electrons in the same shell do not screen each other, therefore , the electrons experience greater nuclear charge.