Many or most of you probably already know this but here goes:
On a Y-connected system with balanced linear loads, at every point in time there is at least one opposing current for every non-zero phase current. So the phase currents (ideally) will cancel at the neutral connection
Nonlinear loads like rectifiers draw current only near the voltage peaks, and therefore the phase currents have short durations that don't overlap. And so there is little or no cancelation of currents at the common neutral point. Therefore the mean square current in the neutral is the sum of the three mean square currents on each phase. So for current pulses with equal value, the total neutral RMS current is sqrt (3) = 1.73... times the RMS current on one phase. If there's any overlap of the phase currents then the neutral current will be less.
I don't think the same worst case condition can exist on the line currents of a delta because there's no single conductor where all of the peak currents flow through. However, there can be circulating harmonic currents in a delta transformer that can cause other problems.
