It increasingly requiers more power to overcome the mass and friction as the input power increases and thus the reason for it being a precentage.
As the input power increase so does the friction that is generated and thus more power used up to overcome this added friction. The faster you try to accelerate something, the more friction is generated.
Think of it as in brakes, (obviously this is the reverse of accerleration, but still the same pricipal is applied) the the faster you try to decelerate a car, the more heat that is generated because the friction was increased. Another way to look at it and can be tested if you do not have ABS is to try to lock up the wheels at 60mph, then again at 120mph (not that I recomend doing this as it is dangerous) but if you did, you would see that it take a noticable more amount of effort to lock up the wheels at 120 then it does at 60. This is because the wheels are inputing more power into the rotors and thus requiers more force or more friction to overcome this power.
Basically it comes down to this, friction changes as the rate of movement changes. The faster you try to move something, the more friction is increase and thus the more power needed or used to overcome this friction.
One other thing to remember or think of is, you can not create or destroy energy, you can only change its form. From this we can understand that friction is the process of changing kenitic energy to heat energy and with this, we can see that heavier power loads (kenitic energy) causes more heat (obviously heat energy) in gearbox's and diffs and thus more friction has been generated. We know that this is true or we wouldn't see the increase in oil temps that we do and race cars wouldn't have a need to run gearbox and diff coolers.
Hope this all helps everyone understand it better.
