Heavy ships . Light ships can be lost from time to time , and eventually you'll have very little of them . Heavy ships ensure that they'll survive a lot of battles . And if used wisely , they can survive almost everything . So basically , if you build heavy ships , you ensure that the crew ( in-game , a crew doesn't matter ) will surive . If they're on a light ( but fast ) ship , something can go wrong ( accidentaly go into primary weapon fire zone , secondaries can kill you too , bombers , a hell lot of fighters ) , and your ship is dead . The crew is dead too , and so you have less people wanting to join the GTVA fleet's , and you have less people to place into new ships . As you will always loose a few of the lighter ships .
By the way , the Sathanas isn't really a good example of the Shivan ships . I would choose the Demon or the Ravana as representers of the Shivan fleet .