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

On moving down the group the atomic radius of Ga is slightly lower than that of Al. This is due to the presence of d - electrons in Ga which do not shield the nucleus effectively. As a result, the electrons in Ga experience greater force of attraction by the nucleus than in Al and hence the atomic radius of Ga 135 pm is slightly less than that of Al 143 pm. 

Thus, the increasing order of atomic radii of group 13 elements is $\mathrm{B} < \mathrm{Ga} < \mathrm{Al} < \mathrm{In} < \mathrm{Tl}$.