This idea would probably complicate implementing ship interiors.. 
My point was that the only fact we know about a cargo rack is its capacity, we don’t know anything about its structure or how it mounts to the ship. Just the maximum mass it can contain.Correct, the internal capacity of an automotive engine has almost no correlation to the external dimensions. I have been building and repairing automobiles and trucks for over 20 years, both light and medium duty.
You’re comparing an internal combustion engine to a pressurized cargo vessel. It’s comparing an apple to an orange.
I will get into the basic physics of pressurized vessels. The inside of a spaceship is pressurized. A large 128m3 volume requires greater wall thickness to maintain shape than a smaller 64m3 vessel; if they are made from the same material, eg. Steel. So technically, having 2x 64m3 internal volume vessels will occupy a smaller external volume than a single 128m3 internal volume.
So to answer your question, yes, the internal volume of an engine is invalid whereas the internal volume of a cargo vessel is valid.
Concidering how trounced you have been here without showing even the slightest inclination towards having the ability to reconsider, I see little point in continuing trying to hammer that nail.It is a suggestion that would require a lot of analysis on FDevs part before anything would be released, not a desperate urge.
Also, how would it be worse? The engineering aspect has brought out imbalances between weapons; Your saying we should remove all of the engineering from the game?
I have over 2000 hours on Xbox before Odyssey. I restarted on PC for Odyssey and currently have 1000 hours racked up, that’s what Steam says anyway. Working on Titan Raijin now, participated destroying 6/8 (well 7/8 so far) of the Titans.Concidering how trounced you have been here without showing even the slightest inclination towards having the ability to reconsider, I see little point in continuing trying to hammer that nail.
But as I'm working on a amateurish hypothesis connected to this, I do have a couple of questions:
How many hours do you have in the game?
Which are you 3 favourite/most played ships, and what is their weapon loadout?
Core modules really wouldn't work for this, for two reasons, even if you just allowed a single swap of one level of size.Don’t need that size 7 sensor?
It's a bit more complicated than that - the "size" of an optional internal isn't really defined rigorously, and "scale" for anything smaller than a planet is really arbitrary in Elite Dangerous.As shown with the 6 vs 7 vs 8 cargo bays, a size 8 is 2x the size of a 7, and a 7 is 2x the size of a 6.
I appreciate your long post and agree with most of what you’re saying. No module has a defined volume, only a mass. The correlation is made because 64 tons of Commodity A has X volume and 128 tons of Commodity A has 2X volume; therefore size 7 cargo rack is 2x the size of a size 6. This same principle is then applied to all other modules. Mass = Volume X density. If the density is held constant (it is in this case), then 2x mass = 2x volume. This is an extremely simplified case so Technically, a size 7 cargo rack is larger total volume than 2x size 6 cargo racks because of the structural support required to hold 128t of cargo vs 64t. I am ignoring this difference and simply stating a size 7 rack is 2x size 6 racks.Core modules really wouldn't work for this, for two reasons, even if you just allowed a single swap of one level of size.
1) You can't undersize sensors or life support currently - or everyone would! A 1D life support has identical performance to an 8D; 1A sensors aren't quite as good as 7A but they're better and substantially lighter than 7D. Similarly, for balance reasons with the SCO fuel consumption compared with typical fuel tank sizes, you can't undersize a SCO FSD at all.
2) Even with the ones where undersizing is allowed, the modules don't have identical performance curves relative to their size number. Increasing the size of a FSD by one level isn't linear from level to level but usually about doubles your range. Conversely, increasing the size of a Power Plant by one level only gives about 25% extra power, and so long as you're within minimum mass at all, increasing the size of Thrusters might only be another 10% speed. So there's much bigger benefits to going in one direction than the other - undersizing the power plant to boost the FSD is a massive bonus (and on some ships, like the Kraits/Python, barely a penalty at all) ... whereas oversizing the power plant by cutting the FSD is probably only workable on ships you never intend to fly between systems.
It'd make the existing weirdness with some of the modules even more obvious. A 2A power plant fits the pattern by being 1/64th of the mass of an 8A. But a 2A power plant has just over a quarter of the output of the 8A. So 4x2A plants are just 1/16th of the mass of an 8A, but more powerful! (You're not suggesting being able to split a core module slot and fit two of the same, but the fact that it would break horribly if you could is also a problem with swapping size levels between modules)
It's a bit more complicated than that - the "size" of an optional internal isn't really defined rigorously, and "scale" for anything smaller than a planet is really arbitrary in Elite Dangerous.
A cargo pod is 2x1x1m, so that puts certain minimum sizes on the volume of an optional internal (which are roughly the same as those implied for passenger cabins).
But the SRV (which fits in a size 2) is much bigger than four cargo pods
And the SLF (which fits in a size 5) is much bigger than 32 cargo pods (and some of them are also bigger than 8 SRVs, at least in terms of bounding box) even before you count the material storage for the flat-packed rebuilds.
But if you assume that the SLF is definitive for the size, or at least sets a floor on it, a size 5 internal should be able to hold far more than 32 cargo pods.
Similarly, if you fit a T-9 out for maximum cargo ... if you piled those almost 800 cargo pods up outside the ship, their total volume would be less than 1% of the T-9's. So what's taking up the rest of the space? (Whatever it is, its mass is barely more than that of the pile of cargo)
The internal space just isn't "real" enough for apparently logical things like 4*2=8 to necessarily be true of it.