David,
That tightening is normal for all neck sizing-only shooting. If you run mild loads it may not occur, but full loads gradually tighten the case fit in the chamber because the steel expands a bit with the brass, but snaps back to original shape much more completely than brass can. At still higher pressure you get to the point the brass can't return to shape as far as the steel can, so the steel clamps down on the brass, causing sticky bolt lift (a high pressure sign).
You can use a
Redding Body die to size just the body while ignoring the neck, which you can leave to your Collet Die. This also allows you to use the die to set the shoulder back not just periodically, but 0.001" to 0.002" every time, if you choose to, which typically gives you still better accuracy for the reasons Bart described.
You can also get a bushing die or other tool to do both operations simultaneous. But if you run the body die first and the Collet Die second, you will tend to have the mandrel of the Collet Die iron out the internal donut ring that can form at the junction of the neck and shoulder, and which can cause pressure increases with a long bullet bearing surface by interfering with its fit.