I took a look at the Spire Cruise and I have to admit I am stumped.
It almost looks like what I have labeled as Small and Medium mounts are actually the same size, they just mount different modules.
Could someone else take a look and see please?
I don't have any datafile trick for finding this out, I just build one in game and see what's available.
The "small" mounts at the front can take on an extra mark of laser cannons, and the "mediums" just behind the shield mount can mount HBS instead and cannot mount that extra mark of laser.
This also applies to the Battleship in which the front can mount mark IV lasers, and the "medium" can mount HBCs.
EDIT: Information in a List:
Cruiser small mount: I-III lasers, I Rail.
Cruiser medium mount: I-II lasers, I HBC, I Rail.
BB small mount: I-IV lasers, I-II rail.
BB medium mount: I-III lasers, I-II HBC, I-II rail.