Then again, perhaps the ship had rockets, warp and
The nature of impulse engines supposedly did not change between Scotty's "Relics" crash and his recovery from it, but there might have been some change involved before that. These round things look quite a bit like the "star destroyer" engines of that ship in the ENT opening credits...
Warp would make small ships practical: endurance requirements would go way down when your interstellar journey only takes a few years rather than centuries, or a few days rather than years, whatever your application.
I wonder if this design didn't become something of a "Skylab" when converted from sublight to warp. That is, the apparent huge fuel or propellant tanks might have been left rather empty when a warp reactor of some sort became the primary consumer of said, and the ship virtually stopped using her sublight engines. Perhaps the tank interiors were converted into payload space?