Volume-wise, it does appear that a Saladin has half the living spaces of a Constitution, engine nacelles omitted, while a Miranda saucer is larger than a Saladin one by half - all roughly matching the quoted (if only outside canon) crew numbers. So we could assume that all the classes are laid out pretty much the same, with similar requirements for the crew, and the difference in numbers directly reflects a difference in size and nothing else.
On the other hand, FJ at least explicitly acknowledged that his various ships fell into different mission categories. A destroyer would probably be staffed differently from a cruiser, in various perfectly logical ways that remain fuzzy to us 21st century laymen.
Then again, I've often thought of the Saladin class as the Trek counterpart to the USN Spruance: a ship built to a ridiculously excessive size for her inaugural mission, so that a projected future would see her loaded with more and different mission gear, budgets allowing. Thus, a cruiser saucer for the Saladin destroyer, just like the Spruance got a cruiser hull that was only loaded with hardware worthy the cruiser description in the later Ticonderoga incarnation.
Standardizing on that big hull probably didn't do as much good for the USN as was the intent. They never built the other growth models, such as Harrier carriers, amphibious assault ships or bombards. Starfleet got more mileage out of the "standard saucer" if FJ and other fan designers are to be believed - and even in canon terms, if we consider the numerous DS9 kitbashes!
Timo Saloniemi
. The aft section is almost as wide as the saucer and only a couple of decks shallower, so it has a lot of space with it. Then with the torpedo launchers in the rollbar, that also removes a bulky piece of hardware, freeing up more internal space. 
