Supply storage was 10kT for 250 supplies, prior to the changes I suggested yesterday. However, in order for the design minister to function correctly the lowest supply storage must have at least as many supplies as the highest engine.
As to why 20kT of storage can only hold as many supplies as a 10kT engine... uuuhm... let's just say that the larger part of the engine (including fuel tanks) it put outside the ship, the 10kT account for the controls and fuselage (or whatever) inside the ship
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I have a couple of more things that I found, I'll post them later (if I just wasn't such a slow writer).
Rollo