That was true in the immediacy of the post WW1 antibellum period. They certainly had an intent to prevent Germany and company from re-arming and to punish them for the War, which was a bit hypocritical given it was in some sense a family feud (everyone's royals are interrelated) and everyone had a hand in the Bloc system that led to the war. The loser always gets terms dictated by the winner.
When relations between nations normalize and thaw, there are always draw downs on the number of supported units. The 'Peace Dividend' is almost always realized in less than 10 years after the end of war. In many cases, that's simple political reality - when in war, you operate an elevated importance for all things military - you grow your forces as fast as you can and they get a much higher part of GDP. Once you are not at war, and usually directly after the end of a war, you vastly cut down your manpower levels (sometimes ship levels come down slower because you can run lighter crews as you don't need as many replacements and ships not going into combat don't need the repairs or as fulsome resupply) and after a while, ship counts drop (they let the oldest or most expensive vessels go first).
The Imperium needs exactly enough units to:
a) Deter an enemy (and in the case of the Zhodani, Imperial Force levels probably drove Zhodani force levels given their desire to just slow and stop the Imperium)
b) Help prevent disruptions to trade by pirates (but that usually only requires the smaller fleet units)
Also, having lower force levels means that you have less likeliness of adventurism that leads to civil wars. If the would-be pretender doesn't have a big fleet presence to draw on, then they aren't as likely to consider coup d'etat.
Of course, the potential desire to coup d'etat plus the military-industrial complex would fight to impede or prevent too many draw downs (because of their financial and other interests).
But given the number of sophonts and local government politicians that would see end of wartime in a region as 'time to invest back into the economy, etc', there's no reasonable way the Imperium could keep charging the wartime rates (and they must have wartime rates because loss rates and operational costs go waaaay up in wartime).
It's a natural trend in history. Look at how every government (even UK, Canada, US....) screw over veterans the minute a conflict is over. All these countries talk about loving and doing right by our soldiers while they are in the news (because of a conflict), but after the war, very shortly too, veterans' care and post-service care get cut and they stop spending as much money on equipment and upgrades and promotions get bogged down, and generally we just don't take as good of care of our veterans as we could.
This trend existed form as far back in organized state antiquity as I can see and still exists. A soldier that is healthy in a war is an asset. A soldier that is injured in a war is something you care for to keep up the morale for the rest of the force. A soldier that is injured or has been discharged after a conflict is a liability to be minimized.
I don't see things like that (as a reserve veteran myself) but that's how governments of all stripes behave. Some are actively hostile to the military and don't do enough for them. Others are pro-military in their pitch, but still don't do enough for them (ah... hypocrisy!).
This is why I think, with limited trade, that the Imperial Fleets would be very different than the huge fleets.
a) If a war takes a few years, which is kind of an eyeblink for a region given space travel times, not 30 to 100 years, then it doesn't make sense to load up so heavily (it'd break the economy).
b) If peace comes again soon, what you'll need are destroyers and maybe a few light cruisers and that's about it to deal with the pirates.
c) There's other places to invest that are of more use to your polity in the long run (grow the economy means more to draw from for the next war).
That's how I get to the notion of fewer, smaller Imperial 'heavy' fleets and more fleets of smaller (yet still large compared to most pirates or PC ships) units when you have the 'light trade' model.
If you have the heavy trade model, yeah, you might well have the money to build all these Tigresses and put a planet's worth of people aboard for a crew. But that's not what I would say applies in the smaller trade situation.
But as you say, it is up to anyone to decide what they believe and what parts of the real world they like to include, while excluding other aspects. The setting is already a mish mash of periods and metaphors that the original designers wedged together which means they have unaligned aspects and they have parts that should behave differently than their historical inpirations.
Tom B