That supplement is riddled with design errors. IMTU it is an Imperial propaganda piece intended to mis-inform potential adversaries...
its not detailed in canon & attempts to extrapolate from canon sources are discouraged by the authors (as not being intended for that purpose) and lead to non-cannon answers.
For much smaller Regimes, you have the choice of Striker, TCS or Pocket Empires.
Are talking MT's fighting ships or supplement 9?
Economics wise, we can infer many things from canon statements about it's economy, eg it's a mixed market economy with a fiat currency without depedence on external trade. It is a huge economy as well, I would think it would be the reason for subsectors, sectors and Domain's; as economic units.
Which is why it doesn't make sense to buy one basttleship if for the same money you can get a full squadron of smaller ships that are individually almost as effective.
Two different, albeit related, discrepancies: Rider vs. ship and cruiser-sized vs. battleship-sized.
We are thinking about X battleships vs. 6X cruisers. The battleships lose.
Except the Imperial Navy is BB's, BR's, CA's, CV's, etc. and then you have to place it vs x threat navy. The only polities that do seem to pose a credible threat are the Hivers and Solomani, Zhodani society doesn't even work, but that's another thread, they haven't been successful in any of their wars in the Spinward Marches, why are they fighting?
BB's make sense for various reasons, one is that that is the way the Imperium does things, large ships to show Imperial power. The Imperial Navy is the Imperium, not just logically, but we can assume it historically from the civil war and Emperors of the Flag and such.
The military budget of Imperial member worlds average 3% of their GNP. Of this the Imperium gets 30%.
A simulataneous attack by the Zhodani, the Solomani, and one lesser threat (Julians, Aslan, Vargr)? Complicated a bit by assets stationed in one part of the Imperium being no use at all in another part.
Budget wise though, you have to figure in the ability to deficit spend, and for years and years. The building of ships is also a way to inject liquidity in the economies of local worlds, so in turn it is growth and employment.
Assets spread out in the Imperium still have the use of being a threat to any of the Imperium's foes. Even a simultaneous attack, only has limited time before the Imperium can gather forces, other sectors will commit their fleets to the fray, for example, in the Spinward Marches how long before forces from the Vland sector reach there? Vland will definitely not want to see the Zhodani on their perimeter, plus the loss of trade. Actually the Imperium would be far more aggressive with her neighbors, just as a matter of how the canonical polities are working, but when Traveller was written, it had become unpopular. But historically, there are a thousand little wars interspersed with a few huge ones, why this would change in the future I don't know. Nor is there any overall governing body to keep the peace.
These are the sinews of war, the economics and doctrine of the Imperial Navy, the threats it willl meet during wartime and it's duties during peacetime. With just giving the capital ships 1 spinal per 100k tons, it increases the threat of capital ships and as Aramis strated, it can absorb more with its BGG's as well. So size is an factor and then there is the psychological factor of keeping the peace between 11,000 semi-autonomous worlds, when the Tigress appears in system, you know the gig is up.
its not detailed in canon & attempts to extrapolate from canon sources are discouraged by the authors (as not being intended for that purpose) and lead to non-cannon answers.
