So, here's what we've arrived at:
The OTU has:
- a ridiculous whack of planets
- a fairly generous defense budget as compared to many other example polities in history
- budgets backed by high pop worlds and high tech worlds
- as a result, large fleets exist, are mobile, are not gutted by politics and tax revolts, and can be used to prosecute large wars with large battlewagons
- can also have those fleets abused to create dynastic crises and eventually the end of the 3I
- can produce resources easily, power easily, and has lots of unfilled space, yet it engages in large scale campaigns and wars that may be hard to see as justified given no shortages of power, resources or space
A small ship OTU that is not going to devolve into festival of swarms would need:
- a physical, economic, of other restriction on the size of ships built
- it could have a different interplanetary travel engine that precludes ships over a particular size
- ...but it would also need a reason swarms won't work (shortage of IP drive materials, alien discovered tech and very much limited in number, no budget to afford swarms, no real reason to exert the effort to create swarms given no lack of resources, space, and power)
- ...and it would vastly change how homogeneous the TU is (lots of trade makes far places likely have more similarities, less trade with smaller ships would tend to produce less homogeneity)
That's about the size of it I think. Limiting # of planets with (decent tech + lots of manufacturing capacity) and # of high tech planets (for funding) would be a big step. A fallen empire might fit that. Also an expansion that was broad but didn't tend to encourage concentration would tend to do that. Political focus that was very planetary (vs. interplanetary) could also limit bigger space military assets/groupings.
TomB