Why should bigger ships have lower speed caps than smaller ones in space? This makes no sense.
It does from an economic perspective:
For a small fast ship you have to spend mabye 10% fuel per second, for a big 100%.
With the medium you can go to shops, get the best offer (stock = near max), go to another shop, sell the best offer (stock = near min), but the huge ship you can't get enough trade items to pay your fuel.
With small you can transport passengers to other stations and these 1-5 passengers will play you a few cr each. Large ships won't get more passengers.
It also makes sense from a balance perspective:
Small ships need to be able to escape, thus you put higher quality items into them, getting this tiny % extra thrust.
Big ships know that no matter how fast they are, there will always be faster ones have the opportunity to choose their angle of attack.
There are less entities in the universe which are a treat and these entities often have the need to reduce their own losses rather than suicide themselves
(would humans have ever evolved so far if they risk a few peoples to die to stronger prey (maybe bee-swarms)?).
And it makes sense from a physics perspective:
IRL we can't build ships >an aircraft carrier. The larger they are, the more they risk to break apart by tidal forces.
We also can't build ion-thrusters for long space ships, because their thrust force is dependent on surface area. Larger ships would have not more profit -> more diminishing returns
We wouldn't build huge ships, because they wouldn't be able to rotate fast enough for the manoeuvres we need them for, thus they would need at least 3-4 90° thrusters to be somewhat use-full and 6 90° to have most of them being able to thrust in the chosen direction
-> dependent on technology maybe too much dead mass.
It also makes sense from a Newtonian perspective that ships are faster than big ones when near them:
If humans stay on a conveyor belt, they don't need own energy to start moving.
Small ships may have similar technologies (tractor rays) which are more efficient are far more compact than using own propulsion.