Kenny,
You might want to look at 250.24, 250.26 and table 250.66 - you can't have that much of a difference in size between phase conductors and your grounded conductor (neutral
That would be true if we were talking about a service, but the original post specified a sub-panel and those sections do not apply. For a feeder the neutral only has to be sized for the maximum unbalanced load. My example of 500 kcmil ungrounded conductors and a #12 grounded conductor is code compliant for a feeder with the types of loads that I specified.
Don
