[dmazur]

After resizing (assuming FL resizing), the neck is pushed forward as the die "reworks" the brass near the shoulder, making the case length longer.

The important dimension for maximizing case life and minimizing headspace separation odds is the head to shoulder distance, sometimes referred to as "cartridge headspace".

Resizing sets this back to factory, or back to a couple thousandths under fired length, depending on what you use as a reference and how you set your resizing die.

If it got longer to the shoulder, it wouldn't chamber.

In general, if you are trimming you are probably working with a bottleneck cartridge (.308 maybe?). Pressures aren't as sensitive to a slight change in seating depth as the change in volume is relatively small, compared to the total case volume.

Pistol cases are something completely different. Setback due to improper tension can cause dramatic increases in pressure for a relatively small amount of setback. But the 9mm case (for example) doesn't have much volume to start with.

I think the only thing you may be confused about is which part of the case gets longer as a result of resizing. It isn't the whole case - just the neck.