why gallium has less atomic radii than aluminium ????and p block atomic radii increases as we go down the group

Aluminium and gallium belongs to group 13.Generally, atomic radii increases on moving down in a group.But the atomic size of gallium is less than aluminium although it lies below aluminium in the group.This because of the poor shielding effect of electrons in the 3d orbital of gallium.The outermost electrons are poorly shielded by d electrons increasing the nuclear attraction on the outer electrons due to which the radius of gallium is smaller than expected. Hence gallium has less atomic radius than aluminium.