This is handled in the relevant .ae files, which you can find under Vehicles\<VehicleName>\<VehicleName.ae> They're a monstrous mess but if you check up the top you should find something like this: (this is taken from LMLP.ae)
The important bit is the line
Small_laser mdt,1,SmallLazer
This defines the Small_laser model to be put on the null mdt, first instance of said null, and then the actual bit you want: the WeaponType used, so SmallLazer.
I just checked and the:
- Small Mobile Laser Cutter uses the SmallLazer
- Chrome Crusher uses the MediumLazer
- Large Mobile Laser Cutter uses the BigLazer
- Mining Laser (Gunstation) uses the ... BigLazer, according to its .ae file? That's very odd, I remember editing the Gunstation WeaponType in the .cfg and that changed the Gunstation in-game. So I'm not sure on this one