The link I provided (above) explains much more than I can in a brief reply here. Neck or Full Length resizing doesn't matter one bit, and chamber pressure has almost nothing to do with this common case expanding problem.
WHY?
This is fairly easy to understand. If you extract a fired case, it will always fit back in the chamber. Right? The problem comes when your case gets resized a few times. After a case has been resized more than 2 or 3 times, it doesn't always re-chamber.
WHY?
New cases usually have .012" to .024" chamber clearance at the shoulder. During the first firing, the case expands to fit the chamber. Right? The new case is then stretched (and thinned) QUITE A LOT, creating an unavoidable "weak spot" above the belt.
THEN WHAT?
After that, any downward pressure on the case causes it to bulge (above the belt) during the reloading process. This repetitive case stretching makes the bulge even easier to occur. Case bulge can be limited (but not eliminated) with accurate resizing.
Keep in mind that if the body of your case is just .001" larger than your chamber . . . . it simply will not fit in your chamber properly.