This is because the atomic radius of an element generally increases as you go down a group in the periodic table. This is because the number of electron shells increases as you go down the group, and the electrons in the outer shells are less strongly attracted to the nucleus. As a result, the atoms become larger.
Bromine is in the same group as chlorine (group 17), but it is one row below chlorine. This means that bromine has one more electron shell than chlorine. The extra electron shell in bromine makes the atoms larger than those of chlorine.
