Without getting into a lab and understanding what the primer manufacturers do, we can do this simple method to determine, generally, which primer cups are harder to deform, or flatten.
A measurement of .001 for Fiocchi means simple that the primers top area, that flattens, after it was fired, expanded by flattening out to .001 inch larger.
Federal Primers expanded .002, or 2 thousandths of an inch, and we see that the CCI SP #500 barely expanded at all, just half of 1 thousandth. The hardest, or thickest perhaps, of the bunch was the Rifle Primer. Does this surprise anyone? It does not expand AT ALL, and the fired primer, was the same diameter as the unfired primer.
So we can correlate from this that the softest primers, that expand the easiest, are Federal, all kinds of Federal, and the Winchester as well.
This is preferable for reloading because we believe you can more clearly understand pressure with softer primer than expands/flattens more easily. Well, you can understand pressure "sooner" because it will flatten under less pressure, to be precise?