Despite many similar hull components with the Galaxy, I envisioned the Nebula-class to have some substantially different interior arrangements. Whereas the Galaxy was designed with long-term space exploration of the Federation's frontiers in mind, with a duration of several years between necessary resupply, it would seem to me that the Nebula was designed to provide defensive and exploratory services largely within the boundaries of the Federation itself. Of course, it would have complete capabilities to serve in a wider galactic exploratory role as the Galaxy does, but I don't see that as the primary intended role of the Nebula.
Unless there's other variants of the mission pod that I haven't seen, I am thinking that there are two separate versions of the Nebula...the triangular pod, denoting a dedicated combat version (henceforth, Combat-Nebula), and the oval pod, indicating a dedicated explorer and science version (Sensor-Nebula). Mission pods are interchangeable between the two variants, of course, but the internal fittings of each ship make the best match when fitted with its intended pod type. Since we've seen many more triangular pod-equipped Nebulas than oval pod ones, I tend to think there are significantly many more Combat-Nebulas in service than Sensor-Nebulas, and I don't think we've ever seen a Sensor-Nebula engaged in combat outside of TNG's "The Wounded". And since both Nebula designs have separate primary functions as I see it, it stands to reason that the interiors of the ships have been constructed to more fully support those specific missions, and of substantially shorter intended duration than the Galaxy, as opposed to the spacious quarters and family-friendly multiple amenities and catch-alls of a multi-year explorer. Out go things like the Arboretum and a mega-sized shuttle bay (at least in the earlier versions; it would appear that the Bonchune CGI has one) and multiple impulse engines for saucer separation (as well as all of the volume-consuming apparatus involved with a now-unnecessary saucer sep capability, like docking latches and overlapping structural integrity field generators); in go items like more torpedo magazines, phaser strip crystal fabrication facilities, troop quarters for a wartime footing, and more sizable cargo bays. The capacious primary hull of a Nebula of either type would have plenty of room to ferry citizens, colony settlers, and cargo across the length and breadth of the Federation with great economy as Nebulas go about their concerns, especially if the hull did not have to have multiple science labs and sensor emplacements and support of same like the Galaxy does. Additionally, a less specialized primary hull with more cargo space and less "busy" than a Galaxy's requirements would be easier, faster, and cheaper to fabricate, and might serve as an in-universe explanation for the lack of window rows and other visual differences from the Galaxy-class as seen on early Nebulas.
If the Galaxy is a "let's see what's out there" ship, the Nebula is a "now that we know what's out there, here's the ship to deal with it most efficiently" ship.
Combat-Nebulas, I perceive, take advantage of their less-than-Galaxy size to create a smaller, stronger shield envelope than a Galaxy would have from the same sized warp core, and hence would likely be able to stand up to stronger punishment than a Galaxy against the same foe. A dedicated mission pod with multiple torpedo launchers (and there's plenty of room in the pod for a dozen launchers and thousands of photon torpedoes) would be complemented by additional torpedo magazines and other combat-support facilities within the primary and engineering hulls, as stated above. Combat-Nebulas on patrol at key trouble spots within a few weeks or months of the nearest Starbase could, in principle, remain at the ready nearly indefinitely, being relieved by another Combat-Nebula as is needed to return to a Starbase for resupply, crew rest/replacement, and other things. Additionally, a high cargo/passenger capacity would be of great value moving things around within the Federation, along with the defensive capabilities to protect them.
Sensor-Nebulas, though few in number, could be sent on missions of longer duration (though not as long as the intended duration of the Galaxy-class) to places of scientific interest within the Federation (and outside it, with sufficient tactical back-up...). Places like the Dyson Sphere and other anomalies that require more scrutiny that might be outside the capabilities of Oberth- and Nova-class explorer and science ships, or places that a more widely-ranging Galaxy-class might discover and then move along once a Sensor-Nebula shows up or is enroute to the new discovery. Dedicated science labs and analysis facilities would take up excess space used for other functions in the Galaxy-class, though things like the Arboretum and a large (or even expanded, into the interior of the primary hull...) shuttle bay may be retained. Sensor-Nebula also retains capable offensive systems, though not as expansive as that of a Combat-Nebula, making it a better choice than an Oberth- or Nova-class for in-depth scrutiny missions outside Federation space or close to known threats; in that vein, I can certainly see a Sensor-Nebula testing new stuff in the proximity of the Romulan Neutral Zone, for example. Likewise the Combat-Nebula, large passenger facilities and cargo bays may make the Sensor-Nebula a busy enough workhorse for the needs of the Federation when mysteries to be studied are few and far between.
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