Note that it is common to use the word "neutral" interchangeably with "grounded conductor", and 9 times out of 10, these will be interchangeable terms. There is a subtle difference, but it doesn't concern this question.
250.102(C) gives you a lower limit on the size of the neutral, when it is supply-side of the main disconnect in a circuit that classifies as service conductors. Or in a circuit that would be governed by 240.21(C) for transformer secondary conductors of customer-owned transformers. If the calculation of the actual amps on the neutral conductor ends up being negligible, then 250.102(C) governs its size.
Essentially, whatever the size of the green or bare wire in the circuit would need to be, the neutral can be no smaller, whether you call that green or bare wire an equipment grounding conductor (EGC) or a supply-side bonding jumper (SSBJ). EGC's per 250.122 go with feeders and branch circuits that are load-side of OCPD, while SSBJ's are line-side of the highest ranking OCPD on the service (service disconnect) or separately-derived system.
If a much larger neutral current is predicted through your load calculations, then the neutral has to be sized to have an ampacity sufficient for the actual load that would be present on the neutral. Most people just match the neutral to the size of the ungrounded conductors, any time there would be significant current on the neutral.
