Ed, what you experienced is par for the course. Nothing you did wrong. It just happens.
When I first started shooting and reloading, it took me a while to come to the same conclusion as you observed. That the more stout the charge, the less the shrinkage. I suspect It is a function of how tightly the case grips the chamber walls during ignition and rise in pressure. Too little pressure (grip), the more the forward half of the case shrinks back when it expands. Equally important is how quickly it comes up to pressure. If it comes up too slowly, the rearward thrust of the case starts it moving rearward before it finally grips the chamber. That can be seen when the case head imprints from the bolt extractor and ejector.There is a window for this, where after some point, the entire case grips the chamber wall.
There will be shrinkage, regardless of what you do. It is unavoidable. The good news is that the shrinkage is the worst after the first firing. The amount of shrinkage in subsequent shootings is less each time and eventually stops. The shrinkage will not impact performance adversely, but I choose to sort my cases by length as well as the number of times it has been reloaded.
Here's a good way to keep track of how many times the brass has been shot. By sorting your brass by length, you can minimize the variation in the amount of tension imparted by the taper crimp. The die is in a fixed position and it needs to engage the end of every case the same amount. If they're different lengths, that dies will impart different amounts of taper. I sort mine in .005 increments. IE 1.695, 1.690, 1.685, 1.680, etc. If you choose to crimp your loads utilizing a modified Lee Factory Crimp Die that imparts a stab crimp to the sides of the case, below the mouth, then shrinkage sorting is not necessary.
Here's a step by step process for modifying a Lee FCD, complete with pictures.
Hoot