Creating a new hull - or any subsystem, for that matter - is actually pretty straightforward.
I'll give you a quick rundown, assuming you are building upon Galactic Armory 1.9.4.
The subsystem files are located in x:\yourgamesdirectory\Star Ruler\Mods\galactic-armory-1.9.4\Game Data\Sub Systems. Hulls.txt contains, as the name implies, all the hulls present in the mod. If you open the file in any text editor, such as Notepad++, and scroll down you will see a subsystem entry called HaulerHull.
The first few lines look like this:
The first line defines the name used internally for this subsystem. This is *not* the name shown inside the game, though!
The second and third lines contain the name and description for the subsystem as it appears in-game. Or rather, it contains a reference
to the name and description, which are located in the mod's Locales/english.txt. If you add a new subsystem, you will also need to create name and description entries in that file, otherwise they won't show up in-game.
The fourth line tells the game which subsystem icon is used. These first have to be defined in materials/subsys_icons.txt, though.
If I understand you correctly, you want to create a new hull that has more storage but less internal space, right? In that case you need to have a look at these lines:
Size: (Object.Size * min(((10 + floor(Object.Scale / 100)) / 15), (13 / 15))) + vExtra
vCargoStore: Size * 100 * (4 - decay(pow(LEVEL_GAIN_CURVE, Level), 25) - decay(pow(LEVEL_GAIN_CURVE, Level), 125) - decay(pow(LEVEL_GAIN_CURVE, Level), 625))
The current Size gives you a minimum internal space of 10, with an increase of 1 per 100 units of scale (up to a maximum of 13). Let's assume you want it to be a minimum of 7 instead, with an increase per 100 units of scale up to a maximum of 10, in which case it would look like this:
Size: (Object.Size * min(((7 + floor(Object.Scale / 100)) / 15), (10 / 15))) + vExtra
As for the storage space, you could either tweak the numbers in the - slightly complicated-looking - formula or even come up with a (more straightforward?) formula yourself. Feel free to experiment!
(Oh, and if you want to add, say, Helium-3 storage to a subsystem, you'll need to add a provision for H3Storage; have a look at HydrogenStorage in subsystems/storage.txt and you'll see how that works.)
I hope that helps! If there's anything else you need, just ask.