Tom,
Your right, the expander ball does extend the case slightly. But I've found that the effect is only .001" or .002" and that it wasn't enough to prevent the bolt of a rifle closing.

On the other hand, a partially resized case may be lengthened by .010" or more. Or the case may be lengthened slightly. The determining factor is the diameter of the case body area of the chamber of the rifle. The largerthe diameter, the more likely it is that this situation may be created. If, however, the chamber of a rifle is close to, or at minimum, the situation may verywell never happen.

I've gone almost entirely to dies that use sizing bushings to do my resizing with, The exception to this is my full length sizing die. Then I adjust the expander ball differently than most people. I had discovered that it was possible for the
expander ball to pull the neck out of alignment when it was adjusted the way RCBS dies are made so the case neck passes over the ball near the end of the cases downward movement. With the case in the full upward portion on the stroke, I raise the ball until the ball is touching the inside of the case. I then lower the ball about 1/8". I want a major part of the case neck still in the poat of the die that sizes the neck as the case starts over the ball. This forces the ball to be in true alignment with the axis of the neck. Than I tighten the lock ring for the expander stem. This gives me two things. 1, A case neck ine true alignment with the bore of the rifle, if the die itself is true. And 2, the case neck starts over the ball when the press handle is in its lowest position and at the peak of the mechanical leverage of the press. The case is so much easier to move over the ball that I don't get near as tired. Now I know for you young
guys don't need to worry about such things. But it does make a difference for us septuagenarians it does make a big difference. Of course if you adjust the ball in this manner it is necessary to use a universal decapping die to remove primere. I've gotten to the point that I use my decapping die on all my cases and then clean prime pockets by hand.