Here's the basic conundrum:

An imperium based on 'tramp steamer'-scale trade does not need and cannot afford the massive navy and starport infrastructure it has. (See: Trillion Credit Squadron, which has been hailed as canon many times) .

If you have such a volume of trade that you need dozens of 10KT ships regularly plying a route, far fewer 100Kton ships will be more economical. And if you need a dozen of those, well, a 1MDt container ship will make you more money, once costs are amortized (and remember, ships last hundreds of years) because the costs of operation do not scale linearly with ship size.

This is WHY we have huge containerized bulk transports today.

and if you have trade routes that have 1MDt ships traveling them regularly, well lo and behold: you have the tax revenue, and infrastructure needs to build the canon Imperial Navy and starport system.

If the Imperium really is serviced by 200Dt ships once per 10 weeks, the economic value of interstellar trade is simply too low to generate the taxes needed for the IN and the rest of the Imperial bureaucracy.

The only way to accommodate both things as true is to assert that the game rules are for modeling PC-level 'interstellar scraps' trade, not a comprehensive model of the Imperium-wide economy.

I've analogized it in the past as "Trying to model modern global trade economics, based on cross-Indian Ocean dhow cargo trade data."

--
Bruce Johnson

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This. But also, the modern container trade reinvented trade and economies, massively expanding both. I cannot imagine the 3I *not* seeing that. The tramp trade still exists in the 3I at a level it no longer does here, simply because the TU ius so vast, with so many lower tier worlds that it's feasible for the tramps to continue to be primary service for the fringe worlds, as well as the FedEx/UPS/USPS and niche specialty cargo operators of the TU.