Can I downsize the neutral for a 225 amp subpanel feeder?

Yes, within limits. NEC 220.61 allows the feeder neutral to be calculated based on the maximum unbalanced load rather than matching the ungrounded conductor size one to one, which is why you regularly see a 225 amp feeder run with a smaller neutral than the two hot conductors. The calculation looks at the connected loads, applies the demand factors in 220.61(B) for the portion of the load over 200 amps, and the result is often meaningfully smaller than the phase conductors.

What actually limits the downsize

The neutral still has to be large enough to carry any actual unbalanced current safely and to serve as an effective ground-fault current path back to the source, since the feeder neutral doubles as the bonding path in a subpanel scenario without a separate equipment ground, or works alongside a full size equipment grounding conductor when both are run, which is standard in modern subpanel feeders. Never downsize the equipment grounding conductor using the same load calculation logic, EGC sizing follows Table 250.122 off the overcurrent device rating, a completely separate rule from neutral load calculations.

Common field mistake

Confusing neutral downsizing with equipment ground downsizing is the error that gets flagged most. A feeder can legally run a reduced neutral and a full equipment grounding conductor at the same time, they are sized by two different sections for two different purposes, and mixing up the logic on one for the other produces an undersized ground fault path.

Field takeaway

Calculate the feeder neutral from actual unbalanced load per 220.61, downsizing it is normal and code compliant. Keep the equipment grounding conductor sized from Table 250.122 independent of that neutral calculation every time.