I am a big proponent of absorbing lessons from the real world and, as you say, Thomas, there are usually multiple reasons for any big decision. It's like that with wars, religious campaigns, revolutions, etc.
It's never just one thing, despite whatever PR or spin may be put on it to make us feel like we did 'the thing' for 'the right reasons'. Usually there is somebody making money out of whatever path is taken and there are political fortunes being impacted and power dynamics between nations and smaller entities. There are some big events that do have some more 'admirable' reasons (opposing the government and persons responsible for the holocaust, trying to stop genocide, etc). But a lot of what gets blamed on religion even could be traced to secular fortunes of senior church authorities (financial, power related, or for other reasons far from any heaven....).
Your second point is very well conceived as well. You do have to find the balance that will limit larger vessels while still making small ships possible.
I have had two thoughts, not related, as an early musing on that topic:
- It could be that the small ships originate from a government program (possibly still running, possibly defunct) that tried to meet at least minimal communication, mail, and rare cargo movement to and from the far flung frontier planets (or the off-the-trade-spines planets in the more developed areas). The ships would be loaned to the crews, usually a family or tight knit group, with a mortgage to the government. The government might even help in the early days by giving the small guys preferential pricing for some repairs or fuel that would help them get started (and continues perhaps as a modest subsidy for weakly-justified trade paths).
You could work your way to owning a ship, or maybe when they get old enough, they get sold for a fraction of the purchase price (with some 'character' - better have Kaywinnet Lee Frye aboard to 'keep her flying'). That would explain small private groups puddling around with aged ships that more commercial/corporate buyers might have long sold off. It would explain why the fringes only really see this guys too - not enough trade to justify any sort of 'schedule' of arrival or at least, any such trade would be minimal even if scheduled by a contract.
- If it turned out that the commercial shipping (except perhaps for some moving very high value cargo or very high value passengers) is built on one tech base (say one type of drive system that will get you there, but takes a bit of time and it has fewer comfort/entertainment/large living spaces, etc) and military and highly valued commercial traffic could be built from another (a much more efficient drive system, a lot more space for living (civ), a lot of amenities (civ), and a lot more comms/sensors/defenses, etc - much moreso in the military ships). The cost tiers could then be vastly different.
Tom B