The reason why Br has a greater magnitude of electron affinity than that of I is that there is a greater attraction between an added electron and the nucleus in Br than in I.
In the periodic table, there are trends that increase down the group and across the period. Electron affinity is a trend that increases across the period but decreases down the group.
Recall that the ability of an atom to accept an electron depends on the size of the atom. The smaller the atom, the greater the attraction between an added electron and the nucleus.
Since Br is smaller than I, there is a greater attraction between an added electron and the nucleus in Br than in I which explains why Br has a greater magnitude of electron affinity than I.
