It's not as straightforward because one situation is that a population is represented in federal governance as per the general representation of the citizenry. The other situation is not. If they've passed their referendum, finally, requesting the votes and the taxes then they deserve the votes and the taxes, as per the general representation of the citizenry.
If we're going to get into carving existing states into smaller states, I can think of worse conservation-minded policy than giving the senators (mostly) to the West and Midwest. Might even get right to repair. But yes, complicated. We got into a war in name about this once.