That's not how I meant it either.
You only need to go so far, so fast with ships that are doing certain tasks. Thus the absolute hull space needed for say 6 months of travel time is fixed. To put it another way, if I only have to go 3 million kilometers and how long it takes me to get there is a non-issue, then an absolute value exists with regards to how much Hull Space (or tonnage) I need to get there.
Or, put another way; if I just need to pop over to Venus, I only need X amount of speed in km/s to catch up and X amount of fuel to make the trip; let's say enough for a single round trip. Since both values for that trip to Venus are fixed values which don't need to be exceeded, my ship with a Nuclear Pulse Engine can make that speed for less fuel than my Magneto-Plasma version, because the Magneto-Plasma eats more fuel per Hull Space, requiring more Hull Space for fuel to create an amount of speed that is wasted on my little put-put over to Venus.
Basically, if I only need X speed and Y distance, where X and Y are fixed values which do not need to be exceeded, I could use Nuclear-Pulse, Ion or whatever "lower tech" engine in place of a higher tech one to save Hull Space since I don't need the speed of a better engine, nor do I need the range which it would afford me by the merit of said higher speed.
Yes, I know the argument can be made that a higher tech engine gives more of X and Y for less Hull Space by virtue of said speed, but a Cargo Module still weighs a fixed amount, as do Sorium Harvesters, Asteroid Miners, Salvage Modules, Jump Gate construction modules and so on and so forth. Which brings me to the second part of my OP, where the Hull Space itself has a certain fixed value by merit of it's role. A Sorium Harvester will always weigh AT LEAST as much as one Sorium Module; a Cargo Freighter will always weigh as much as at least one Small Cargo Module; etc.
So, with Engine Power Boost tech, and fuel consumption tech, as well as the lower tech engines being less fuel hungry per HS; I posit that it is useful to use lower tech engines for ships that have:
A - A role which demands a certain minimum tonnage which is non-negotiable. I.E. Cargo Modules, Cryo modules and others that don't benefit from size-reductions through tech like weapons, power plants, FCS, Sensors etc as being mandatory to accomplish their mission.
B - Only need to go a certain distance, presumably inter-system, but Jump Drive enabled is valid: so long as the ship is only operating within a certain number of km or time period.
C - Doesn't need to be there under a time crunch. One month or three doesn't REALLY matter for a Colony Ship. Although faster could be better, it doesn't HAVE to go faster to get the job done.
Just for some clarity, I was in the laundro-mat when composing the first piece.
So, in light of this, does my assumption have any merits to it at all?