The average oxidation number (O.N.) of S in S2O32- is +2. Being a stronger oxidising agent than I2, Br2 oxidises , in which the O.N. of S is +6. However, I2 is a weak oxidising agent.
Therefore, it oxidises , in which the average O.N. of S is only +2.5. As a result , S2O32- reacts differently with iodine and bromine.