Don't see how that reduce lag from a lot of ships circling around though since it wouldn't reduce the amount of active ships from the sound of it?
My idea is simple...
If instead of having 10 times as many ships to service 10 times as much traffic each ship carriers 10 times as much cargo there is no extra lag at all lategame.
Basically, you want to have larger ships built later in the game.
The potential problem is that the shipping lines need to buy each ship. Making ships bigger means you have fewer of them overall, and doesn't give you as much money. Why?
First, because money sitting and waiting for an appropriate amount to be collected for a new ship isn't doing anything. A smaller ship bought with the same money is.
Second, things like dividends would complicate buying them. If a shipping line doesn't make enough with its current ships to pay for one new ship a year, it never gets new ships. Then it goes away when they age out. A new line will need massive subsidies to get started.
Third, a big ship is less versatile than a bunch of small ones. If you need one automated mine carried, it's best to do it on a ship with a standard cargo bay. Some early worlds might not supply or demand enough stuff to make a run with a big ship viable, leaving them ignored.
Another problem is that late game is relative. If you do a conventional start, you could have ships coming out of your ears with only ion engines. If you start with ion engines, you might get to magnetic confinement fusion before the problem starts. So the biasing would have to be relative to the current civilian fleet, not an absolute.
That's not to say that some bias in fleet buying is a bad idea. Maybe set it up so that as more ships get built, the bigger ones are more likely than the smaller ones.