actually has about 30% more internal volume than the Nebula
. The saucer and engineering hulls on the ships are not actually the same. The engineering hull in particular is a fair bit smaller, while the saucer lacks the large impulse engines. (In fact the Nebula
seems to have extremely small impulse engines in general.)
Further, in terms of little details the Nebula
is clearly a more "economy" version. The lack of the "neck" precludes the large, advanced 10-torpedoes at a time torpedo tube in the Galaxy
, it has a smaller main deflector, and at least the original studio model lacked windows and the main shuttlebay on the saucer. Even on the CGI model that reuses Galaxy
mesh elements, it's not entirely clear if the saucer shuttlebay is still there. It's possible the whole thing has been turned into a large cargo bay.
In which case the engineering hull is more likely to have a lot of its rear half taken up by shuttle facilities where its main shuttle bay is located. Overall the ship has less usable volume and substantially less expandability.