...The catchall fanwank is that this is an old shuttlecarrier design (such as in the fan works of the 1980s), made obsolete and thus available for other purposes - and, because of her pedigree, has prodigious amounts of internal space for harebrained experiments, installed in weird modules that occupy but part of the former hangar and leave the rest of the space full of, well, nothing much. Except for impromptu turborails and corridors to connect the modules, of course.
The aftmost part of the hangar area has been refitted into a shuttle facility for the much more modest needs of the flying lab, but still retains the launch tubes and turntables of a much busier auxiliary craft operator. The neck connecting the hulls is empty on the inside, too, with a large porthole-free hatch as its forward surface, so that the pressurized corridors on each deck run along the sides, with portholes to space from them rather than from possibly more deserving cabins or labs or whatnot.
Also, there was space for 300 experiments originally. Only the spore drive ever mattered, though (that is, it probably was the only one that panned out as an anti-Klingon Wunderwaffe, and it was the only one the CO of the ship cared about). So a second round of gutting probably ensued at some point, leaving the interior even emptier now that the Green Matter Bomb labs and the Tri-isophasic Swirl Phaser test ranges were removed.
Fanwanks aside, the DSC ships come in a great variety of shapes and liveries, befitting of a fleet consisting of ships of varying ages. This matches TNG very nicely, in a fictional century removed by, well, a century. And helps establish a pattern that persists in shows separated by decades ITRW. What is our excuse for the pattern breaking down in TAS? Obviously the low number of designs seen overall. In LDS? So far, the same, I guess. All the other incarnations of Trek are doing just fine in that respect.
Timo Saloniemi