We can get around this by looking at what canon says the Fleets are made up of and inferring rough support capabilities. Mike Wightman and Steve Osmanski have done some of this, I think, and I've tried, as has Hans Rancke.
Some parts of canon should be considered more relevant then others, imo. I'm pretty good with business/ecocnomics (I received a 100% on a final in it recently, something my prof said nobody else has done), it's economy is immense, even cut into zones, the way the Chinese do it today. Some addendum to canon could be added to explain things:
1) The Imperium builds BB's the way others build CA's, back to the "this is the way we do things in the Imperium", large and in charge.
2) Supp 9 and other sources like SMC, FFW, JTAS articles, etc.; only cover the Spinward Marches and the fleets in the Marches are kept purposely weak to prevent any Emperors of the Flag civil war type occurances. A Tigress in the Core Fleet may be a BGG carrying multi-spinal super-dreadnaught, but they pull armament and the force field when they send them out to the colonies. This may be well known, just not documented as the Imperium isn't going to advertise the fact.
3) BB's as they stand right now are in fact "Treaty BB's", not that there is a specific treaty, but if the Imperium started building fleets of multi-spinal BGG protected BB's, it will start an arms race with the other polities to catch up.
The first three frontier wars were successful for the Zhodani. The Imperium doesn't realize it, but the Zhodani don't want to conquer the Spinward Marches. What they want is to deter the Imperium from expanding any further in their direction. Imperial settlers were pushing into Foreven at the time of the First Frontier War.
Then they are doing it very wrong, because the people of the Marches are going to see the Zhodani as the main threat, which will force the Imperium to act in order to keep it's reason for being. The Imperium can also absorb Zhodani worlds, something the Zhodani can't do in turn and the Imperium can also destroy the Zhodani Consulate if it wanted to.
A few large ships can be justified this way, but not hundreds and thousands. Using them to impress the member worlds (or at least the yokels on the member worlds) can be justified; organizing then in squadrons and putting them in the line of battle can not.
If one has the Credits, why not? The Imperium has all the Credits they ever want, they control their currency. If you can build BB's the way others build CA's and still out number their CA's, then part of it is psychological warfare, don't mess with us or our dreadnaughts will swarm your puny worlds; eg deterence.