There was a time I ask "It it stretch or flow, or is it stretch and flow, or is it the same, stretch is flow??
Nothing! As to your findings by measuring before and after, I am going to assume you are talking about case length, that would be from the case head to the mouth of the case, if so that is what I would expect. When the case is fired (depending on the rifle) the case fills the chamber, most shooters have fast ammo meaning the case, bullet powder and primer accelerates to a speed that allows it to get to the front of the chamber before the primer is crushes by the firing pin, not my ammo, my firing pin crushed the primer before the bullet, case and powder knows their little buddy the primer has been hit.
By what ever method the case when fired fills the chamber, by what ever method the shoulder forms to the shoulder of the chamber, the case body fills the space provided for it in the chamber body and the neck expands, and, the head of the case moves back to the bolt face, every event described contributes to the case stretch two ways if allowed, out against the chamber walls and out as in forward.
Then comes full length sizing to minimum length, the body of the fired case diameter is reduced, when the body is reduced in diameter the shoulder moves forward and takes on the appearance of a radius, when the shoulder is pushed forward the neck also moves forward (case gets longer from the head of the case to the mouth of the case), at this point the neck gets sized, it does not get thicker or thinner, it gets longer or shorter, when the neck is sized down the neck gets longer, when the neck is expanded the neck gets shorter, so when full length sizing expect the case to get longer.
Back to my assumption, the case getting longer when sized is a given, my purpose for measuring a sized case is to determine the length of the case from the head of the case to it shoulder, meaning if I am full length sizing cases to minimum length the length of the case will shorten from the head of the case to it's shoulder. I determine the length of the chamber before sizing and my favorite case is one that will not chamber because it is too long from the head of the case to it's shoulder, and that is what I do, I size cases to the chamber, others fire first and call themselves 'fire formers' when I fire a case I get once fired cases,
Difference? others fire to determine the effect the chamber will have on a case when fired, I measure the chamber first to determine the effect the chamber WILL have on a case when fired. After firing is a good time to gain information about the effect the chamber has on fired cases, I am not a fan of making the same mistake over and over and over, fire it once, then measure, firing THAT ONE CASE 5 times leads the reloader into the twilight zone, they think the case is becoming an adult case and is not full grown until it is fired 5 times +, fire it once, make your measurements, and if able and have access to the companion tool to the press, the feeler gage, make adjustments.
Learn to use the FL sizer die before you allow someone to talk you into getting a 'not necessary but nice to have die.
F. Guffey