Is 220.61(C)(1) stating that you cannot reduce the neutral size in a 4-wire, 3 phase, wye system? Does this mean that the Neutral (grounded conductor) must be at least the same size as the ungrounded conductors?
Read it carefully, it says when using two ungrounded conductors and the neutral of a wye system.
In that situation because of the 120 degree phase angle the neutral doesn't carry imbalance current of the phase conductors like it does with a 180 degree phase angle. If you have 10 amp line to neutral load on A and 10 amp line to neutral load on B the neutral will carry approximately 10 amps as well, but with same connected load in a single phase application with 180 degree phase angle that neutral is not carrying any current.