Hull hardness isn't a "hidden fiddle-factor", it's displayed in the statistics tab on your right hand panel, is a ship specific value and directly affects the hull heath of a ship when a weapon hits it.
It makes the fact that smaller ships appear to have stronger integrity pointless because that Saud Kruger liner may have more "integrity" than the top Imperial warship, but that warship takes substantially less damage from all weapons that don't have a high armour piercing value.
Unless you're also saying a weapon's armour piercing doesn't matter since it's also a "hidden fiddle-factor", even though it's front and center in the outfitting and is also on an equipped weapon's info page in the modules tab.
The point is that, by way of analogy, if you take 2 identical beer cans, you can't make one of them tougher simply by "increasing it's hardness".
If you wanted to make one beer can tougher you'd have to either add more material, construct it from a different material or construct it differently.
When you do that, you're going to change a variety of variables in the process.
You're going to make the can heavier (as a result of adding more material) or more expensive (as a result of using a different materials or different construction methods).
The "toughness" is simply a function of the construction criteria.
Most importantly, you're going to end up with a can that
is tougher than the standard one.
Again, don't get me wrong.
A value for "hardness" is a reasonable way to
describe the result of all the construction criteria but it shouldn't be a variable itself.
You shouldn't be able to have a regular beer can with a "hardness" of 10 and then say "Okay,
this is going to be a combat beer can so we'll just give it a hardness of 20 instead".
More importantly, with proper mathematical modelling in place, you wouldn't
need to do that because you'd simply twiddle the other variables (material, thickness, construction method) until you
did get your desired "hardness" value.
It's really not a big deal to implement either.
If somebody had access to all the data for ship surface area and volume (and any other "hidden" stat's FDev rely on) it'd only take a couple of days to knock together a spreadsheet that'd allow you to look at a ship, pick from a variety of predefined construction materials/methods and then adjust the amount of material used to obtain the desired properties; weight, integrity, hardness etc.
Don't forget, as well, that modules are an entirely separate issue too.
If for example, somebody looks at the model for the AspX, pumps all the numbers into a spreadsheet and discovers that really
should weigh 400t and, thus, only be able to jump 35Ly, that isn't a problem.
If the AspX is intended to be a viable exploration ship, it just gets given a bigger FSD.
*EDIT*
Maybe worth saying, it might seem like I'm fixating on the issues surrounding hull design rather than anything else but that's not the case.
It's just that getting the hulls consistent seems like the first step required
before you can look at things like internals - especially in regard to core modules such as thrusters and FSDs etc.
On that note, one thing I
would say is that, having watched a bunch of video's on the subject, most people seem to be assuming that internal components will always scale linearly with class and I'm not sure that's always wise.
In the case of optional internals, a simple "rule of thumb" might be that all slots need to be physically large enough to contain the physically largest module that could fit in it.
A C6 limpet controller, for example, might only be the size of a golf-cart but a 6E passenger cabin
has to be big enough to support 32 people, which means that
every C6 slot has got to be at least 64m³ in volume.
On that basis, the physical size of optional internal slots
is going to increase linearly.
4E cabin; 16m³, 5E cabin; 32m³, 6E cabin: 64m³....
However...
The same isn't true of core internal slots.
There's no reason to assume that, for example, a C6 Power Plant would be twice the volume of a C5 PP, or 4 times the volume of a C4 PP etc.
By way of analogy, the 250hp engine in a hot-hatch fits in the same engine bay as the 110hp engine in the econobox version of the same car.
It'd be pretty easy to just make the "engine room" of a ship in whatever size was convenient and then have 2 or 3 different 3D models for PPs and bung in whichever one was the most suitable representation.
Same thing for pretty-much
all the core internals.