It honestly wouldn't surprise me if future revisions of NEC mandate neutrals LARGER than that of the phase conductors to account for the continuous harmonic current the neutrals are subjected to.
If the two hots are 180 degrees in phase apart, you can get away with normal or undersized neutrals. The timing of the harmonic current draw on one such phase is the same for the other phase. Which means that the harmonics on the neutral would cancel (if equal). Where the problem comes up is in systems where the phases are 120 degrees apart. Then you will not get cancellation on the neutral, as the timing of the harmonic currents are different.
