Rocket engines aren't necessarily all that long. There's plumbing to the combustion chamber, which can be mounted radially, the chamber itself, and the exhaust bell, which on real spacecraft makes up the majority of the length.
https://i.pinimg.com/originals/23/6b/2f/236b2f5ca995b4b96ea036440be8bbf4.jpg
Most sci fi ships ignore the bell, or use atmospheric bell shapes which are shorter, or sidestep the issue altogether and use ion/plasma propulsion, which wouldn't need a bell.
https://www2.jpl.nasa.gov/files/images/browse/ioneng1.gif
Elite seems to mostly use the ion engine type, which could be remarkably compact units, where most of the "engine" mass would be in plumbing and power tranfer cables, capacitors and the like, which could be mounted anywhere, really.
Y'know, that is all very plausible, and I've often assumed that ED ships use some kind of ion-drive myself.
The only trouble is, if that's the case, what
is taking up all the space inside a ship's hull?
We know it's not cargo/passenger/crew space cos that only accounts for a tiny percentage of the internal volume of big ships.
I guess it
could be the PP, shield generator and/or FSD but the configuration of ships like the T9 and T10 (ships with "outboard" thrusters) suggests that the thrusters, themselves, are big ol' assemblies.
In the case of the Chieftain, if the thrusters
aren't filling up those pods, why are the pods (the front ones in particular) there at all?
If it's the gimbal mechanism that's taking up the space, and requires the pods, we're back to wondering how the landing gear fits in there as well.
For the most part, as long as a sci-fi ship looks cool then I'm happy but it's that little extra bit of plausibility that really makes a thing believable.
Seems like it might help if FDev got one of their artists to make some basic sketches of components such as thruster assemblies, FSDs, PPs and shield generators etc in, say, small, medium and large sizes to use for reference when ships are being designed.
I only just discovered this but, on a related topic, the producers of Star Trek write a "technical manual" for each series, which contains basic info' about what all the in-universe tech can and can't do.
The manuals aren't detailed, and they don't explain
how anything works, but they lay down basic rules so that writers and designers can always come up with stuff that abides by all the rules that pre-exist in that fiction.