If you don't need to set aside space for large jump fuel tanks you can use that tonnage for more offensive and defensive armaments. I've never agreed with the rules that tiny 400tond SDB can have equivalent armor to a Tigress dreadnought. Armor and hull structure just don't work that way. The larger ship has much more plating and structure to support said plating. An SDB would be nothing but armor if it tried to mimic it.Except in Traveller, unless you are looking at J-4+ ships. System ships can have just a many hardpoints and just as much armor as SDBs. I see this written over and over again in the fluff, but it does not seem to hold up mechanically.