Lets take an example Na i.e. Sodium atom. The ionic radius is smaller than atomic radius, because in atomic condition the last one electron occupying an extra shell. When the outer last one electron is released then sodium becomes ionized and becomes smaller in radius than atomic condition. However in case of chlorine atom there is no effect because there is gain of electron to be ion and the gained electrons occupy the position already in the outer shell and needs not to have extra outer shell.