Aircraft Design

[zgrssd]

Observation - Engine Size limit

Increase of the Engine generally increases Range.
However, there is a point where increasing the engine will not increase the Max Ground or Max Air speed - it simply stays the same. Like it hit a limit. A maximum HP:Weight Ratio.
At this point, increasing the Engine will only decrease Range via extra Weight. You have to start increasing the other things (like Fuel Tanks) to get any more range out of them.

So speed and fuel have a role in maximum Range.

[DeltaV112]

Observation - Engine Size limit

Increase of the Engine generally increases Range.
However, there is a point where increasing the engine will not increase the Max Ground or Max Air speed - it simply stays the same. Like it hit a limit. A maximum HP:Weight Ratio.
At this point, increasing the Engine will only decrease Range via extra Weight. You have to start increasing the other things (like Fuel Tanks) to get any more range out of them.

So speed and fuel have a role in maximum Range.

The math used to derive economical speed and range is very complex, and involves a lot of branching options depending on the ranges of parameters. There is no real way to represent it as a math equation. The practical thing you're observing is that aircraft range is dependent on economical speed, which is a fraction of maximum speed if it is much faster than takeoff. That's capped to critical speed, that doesn't depend on engine power. There is a slight impact of power after that, but not much.