If the cases are an interference fit, shooting them seldom makes them smaller.
When cases are fired they get larger. So does the chamber. After the pressure drops, the chamber is back to normal dimensions, and the case shrinks a little due to springback. If your case was an interference fit to begin with, when it "springs back", you will often have even more of an interference fit.
This is another example of not using a case gage to set up your sizing dies. At best you are following the "size to the shellholder and add an eighth turn". This advice is so bogus.
At a minimum, buy a cartridge case gage and size to gage minimum. You will find that pot luck, once fired cases, don't necessarily size to gage minimum. That's why I keep a case gage on the bench and check mystery brass. If the stuff comes out too large, I set my small base die to size a bit more.
Also, I will bet that you are using standard sizing dies.
Here is an example of two military cases fired in a Zepplin sized chamber. One the left is a Wilson gage, on the right a reamer cut gage. The reamer cut gage was a barrel stub cut to depth with a chambering reamer. (My gunsmith did this for $35.00) The Wilson gage only measures length, but the reamer cut chamber will tell me if my cases are too long, and "too fat".
One case sized in a standard sizing die: It is an interference fit after sizing.
The other case sized in this Small Base die: It drops in all the way.
You will find examples of brass fired in chambers so large, that even a small base die won't bring them back.