why ionic radius of Sb is less than Bi ?

Bi is the placed bottom to Sb in p-block of N-family. As we move down the group, the ionic radius generally increases. That is why Bi has larger ionic radis than Sb. The reason for increased ionic radius for higher member of the group is that, filling of electrons takes place at higher energy level. So outermost electrons feels lesser attraction of nucleus hence size is larger for the element with high atomic number in same group.