Was wondering the same thing. From a utilitarian perspective, I keep a few things in mind.
No windows -- I'm assuming that means that whatever is in them, isn't made for habitation or everyday work.
I also keep in mind they're secured to the rest of the ship by relatively skeletal bracing. While thin skeletal structures does not necessarily mean structural weakness or a sign of expendability in Star Trek (D'Deridex Warbird comes to mind), the Dominion has little artistic sense or culture to it. Its ships are primarily designed with utilitarian purposes, expendability, and intimidation.
I first thought that the pods were the hangars for Jem'Hadar fighters, with the larger ships being used by the 3 bays at the fore. But I'd imagine even hangar bays would have some kind of windows for control centers and flight deck operations. So the idea they were used for hangars kind of seems out of the question.
At the same time, however, the two hangar doors on the fore sides of the ship don't really work. They're literally just doors. The thin hull structure it's connected to doesn't go to the rest of the ship (panning the camera you can see the back of the door). Which left the primary hangar door just above the deflector as what I'd imagine is the hangar. But then if it has two hangar bays, the side doors are the only conclusion -- despite the fact they're not connected to any type of flight deck.
Which again made me wonder if those pods were the Dreadnought's hangars. But given the other evidence, I can't say that they are -- even taking into account the flaw of the fore starboard and port hangar bay doors not connected to a flight deck.
Given the skeletal bracing and lack of windows to indicate any sort of important personnel there, I do have to agree with the assessment they're more than likely cargo facilities.
The rest of the ship would logically be inhabited by the troops, any important, non-expendable cargo such as Ketracel White. Any other space would be taken up predominantly by armories, training facilities, crew quarters (for the vorta and any founders), detention centers, war rooms, conference rooms, interrogation rooms, engineering, and a sizable portion being the internal flight decks.
Considering the Dominion's approach on expendability, the pods are likely there to contain captured or derelect ships. The lack of windows and skeletal bracing could indicate those extremely large pods could be used to contain fighters, shuttles, and even frigates and birds of prey. The Dominion also studies their enemies very carefully, so a good number of the pods could efficiently be used to contain salvage and wreckage for further study. In a fight, however, those pods would be considered non-essential to victory, and would be used as a type of ablative armor for the primary hull. Torpedo and energy attacks to those pods would be destroying wreckage and captured ships and their crews, while leaving the primary hull underneath undamaged. As non-essential parts of the ship, it would be a good explanation for their purpose on the dreadnought.
Captain Watters' overconfidence, however, proved to be the Valiant's undoing. Rather than returning the valuable information to Starfleet, Watters decided to attempt to destroy the battleship by exploiting a perceived flaw in her construction. Vastly outgunned, the Valiant was quickly devastated after an unsuccessful attempt to destroy the enemy's antimatter storage system primary support braces. With Captain Watters and most of the bridge crew killed, Nog gave the order to abandon ship. Two of the four escape pods launched were destroyed by the Jem'Hadar ship, while a third was destroyed when it was unable to clear the exploding Valiant. The remaining pod was able to flee the battle area and was eventually recoverd by the Valiant's sister ship, the USS Defiant.