I mean the rim/case head diameter is the same and the overall taper is nearly identical. So if you load 9x19 to the published 9x21 oal then you would have the same effective case volume.
Case volume of an unloaded piece of brass has no real bearing if overall length is the same on effective case capacity.
Example 260 Remington and 6.5 creedmoor when both are loaded to the same oal have approximately the same velocity potential even though the 260 has more case capacity. That is because if you are restricted to the oal of a short action magazine the bullet is stuffed into the case quite a ways. Now if you are running out of a long action mag you can seat at a longer oal and gain enough case capacity to push it ~125 fps faster than 6.5 creedmoor can achieve.