The guardians have varying hull types, like starships now do. It's literally all over the place, same as with the fleet ships. The idea is that there are various ship sizes, conceptually: large (starships), medium (guardians), and small (fleet ships), and they're all just a mix of various hull types, etc. The sizes don't have any effect on anything combat-model-wise, it's just a matter of how powerful those ships are in firepower and health, etc.