I discussed this with Kirsten a bit, the ship had to be long to house everything, but not too wide to make Slipstream impossible. The Mulciber class has rather classic "cargo freighter" layout. Probably Achilles is the only 1 of it's class that has been retrofitted with QSD systems and oversized nacelles.
What I wrote to Kirsten in 2010:
I'm excited about the Achilles having a "carrier" role also. I guess the shape of the Achilles would be more boxy with a lot of bay doors on both sides. Maybe something along the lines of the Karemman freighter from an episode of DS9: practical while still having some kind of "slipstream narrow" appearance. I'm pretty sure that the current canon ships do not meet up to the required specs and internal volume. So this probably would become a new design. In size I'm thinking about 800 to 1000 meters.
I agree that the Achilles was already in service before project Full Circle. It seems impossible to plan, construct and test such a large & critical ship in such a short period of time. I changed the registry number into NCC-77024 to reflect this.
For me the order of construction/design is now:
Final goal is to have a poster (a massive poster) with the entire fleet on it.
Getting Vesta class in Star Trek Online took a lot of energy and I'm currently working on a non Trek ship, but will return to Demeter asap.