How would you explain the lower atomic radii of `Ga` as compared to `Al`?

Explanation

Due to poor shielding of the valence electrons of Ga by the inner 3d-electrons, the effective nuclear charge of Ga is greter in magnitude than that of Al. As a result, the electrons in gallium experience greater force of attraction by then nucleus than in Al and hence atomic size of Ga (135 pm) is slightly less than of Al (143 pm).