I remember reading something on this a long time ago. If I remember correctly it has to do with the resistance per foot of the conductor. The smaller gauges have much higher resistance per foot of conductor than the larger sizes. With a routing/length difference the differential current on the paralleled conductors would be much higher on the smaller wire. I will see if I can dig up the source. If I remember correctly 1/0 was selected based on some determination of acceptability vs likelihood of install issues, after all you will rarely have the exact length on all of the conductors. But that was long ago in a galaxy far away and my memory may be tricking me....
In this case where both neutrals are sufficiently sized for the breaker I do not see an actual safety issue that I would lose sleep over. But it would be a violation of 310.4 and by the way EGC's are covered in 310.4(E):
(E) Equipment Grounding Conductors. Where parallel
equipment grounding conductors are used, they shall be
sized in accordance with 250.122. Sectioned equipment
grounding conductors smaller than 1/0 AWG shall be permitted
in multiconductor cables in accordance with 310.13,
provided the combined circular mil area in each cable complies
with 250.122.