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

Atomic radius (in pm)
Aluminium 143
Galllium 135
Although Ga has one shell more than Al, its size is lesser than Al. This is because of the poor shielding effect of the 3d-electrons. The shielding effect of d-electrons is very poor and the effective nuclear charge experienced by the valence electrons in gallium is much more than it is in the case of Al.