I did an analysis of the marginal improvement of ratio of armor to internal volume (technically tonnage in aurora) on the old forum. See chart below. Basically smaller ships are inefficient (to achieve the same number of layers of armor at a given armor tech level), but the rate of improvement in efficiency really slows down somewhere between 20 and 40k tons.
That analysis did not include the marginal cost of things that don’t increase in size as ship size increases, e.g. sensors, bridge, CIC, etc.
I generally build my largest combat ships around 60k ton, as that, in my experience, is the sweet spot that balances ability to divide your forces to cover multiple areas vs staying power in a sustained fight. If your entire navy tonnage was concentrated in one ship, you can only ever be in once place at a time. If your ships are all super small, they are inefficiently designed (more space taken up by things you only need one of which have a fixed size, again e.g. sensors/bridge, and armor to tonnage ratio). Also, a navy of smaller ships starts losing firepower as each small ship gets penetrated, damaged, and destroyed sooner than less ships of larger individual tonnage.
