[8cd03gro]

Loss to heat as a percentage of power output decreases as power increases because inertia of the drivetrain and friction remain the same regardless of power output. This does not mean loss is a constant because some forces are variable, but the impact of constant forces become less apparent as power increases. These are just hypothetical and in no way accurate as it leaves out a loooottt of variables, but as an example

Let's say I is the force required to overcome the inertia of the drivetrain, it is a constant. Y is the total power output of the engine and X is the power at the wheels. Just for simplicity we will assume a 12% loss due to variables other than inertia. In this oversimplified example the function of total power output to determine wheel hp would be

X = 0.88Y-I

As Y increases, I remains the same. So, let's say I is 10hp, and Y is 400hp.

X = 0.88(400)-10
X = 342 or 85.5% of crank horsepower.

Now we'll increase crank output, Y, to 500

X = 0.88(500)-10
X = 430 or 86% of crank horsepower.

As power increases, power at the wheels will approach, but never reach, 88% of crank horsepower.


Now, again this is massively oversimplified, but you get the idea. Some of the factors that drain hp are constant and some are variable. As you increase power, the constants have less of an impact as far as percentage loss is concerned.

This is much better explained using calculus, but I figure this would be easier understood for some. Hope it helps.
Any engineers, feel free to correct me. My expertise lies in econometrics, not engineering.