Among the III A group elements, the difference in the atomic radius is large in between

B - 1s2 2s2 2p1Al - 1s2 2s2 2p6 3s23p1From B to Al as the number of inner shell s and p- orbital electrons increases,shielding effect increases .Since s and p-orbital electrons acts as strong screens hence they can protect the valency electrons from the nucleus effectively,so nuclear forces of attractions on valency electrons decreases,therfore size increases largely from B to Al.