Comparing the performance of a long, short or belted case of the same caliber. When the goal is to reach maximum performance.

To properly load cases of different capacity, same caliber. The only constant that could be attained would be the bullet weight.
As an Example. The 308 win, 30-06 Springfield, 300 wsm and 300 wm can all be loaded with the same 150gr, 165gr, or 180gr bullet but, have different working pressure and capacity. The working pressure, case, primer, and powder would be variables.
If the best load for a smaller capacity round can exceed the performance of the best load for a larger capacity round. It would be fair to compare the perfomance.

I've been reading that it would be unfair to compare cartridges of the same caliber unless, the same bullet and powder are used. The same bullet is understandable The powder and primer no. Case capacity determines burn rate. Hotter primers burn the powder in larger capacity cases more efficiently.

Hence, if the smaller capacity round can be loaded to exceed the performance of the larger capacity round. It would be a fair to say that the smaller capacity round can out perform the larger capacity round. Assuming the same bullet is used.