Well if you squeeze the diameter back down that brass doesn't dissappear... it has to go somewhere.... it goes back to length. At least IMHO.
That's my take on it too. Squeeze down on a length of any round material, like dough, and it goes somewhere, like out the ends.
I don't like short brass, so I usually prefer to form cases from a larger version of the same cartridge, like 7mm-08 from .308 and .338-06 from .35 Whelen. Gives me some extra length to play with, trimming for an exact fit to my chamber. Of course you have to make sure that the neck doesn't end up too fat for the chamber, but I never experienced that.
I believe that more lengthening occurs from FL sizing than from dragging an expander back through the neck, but it's easy to test.
Paul
