Nutral wire carry the differential current of the 3-phase when there is unbalanced loading on the 3-phase. Mathematically, In = Ia+Ib+Ic, which is a phasor addition and not to be read as arithmetic addition. The nutral current can at the most can be equal to the phase current when only a single phase is loaded. Practically this never happen and the unbalance in the load is small current comparison to phase current and so they have a less cross section then that of phase wire. Practically cross section of nutral is half then phase wire.