Busagringo, this can get pretty deep, but there is a valid and simple way to look at it.
An engine is basically an air pump. Obviously there must be a fuel system to mix fuel with the air, but it is the amount of air moving through an engine that has the greatest effect on power.
You can get the same desired amount of air moved by having a large displacement engine running at low revs, or a smaller engine running faster. Theoretically, you could get the same air movement and the same power from a 400cc engine running at 15000rpm as a 1200cc engine running at 5000rpm.
In the real world it aint that easy. Losses to friction go up sharply with increased rpm, and it gets more difficult to get the air in and out of the engine and supply spark and fuel precisely as rpm increases.
An air and fuel system designed to operate efficiently at the 15000rpm of the 400cc engine would be far less efficient at the 5000rpm peak of the 1200cc engine. the smaller engine would feel "peaky" and would feel gutless at lower speeds despite making the same maximum power. The 1200cc engine would have good power right off idle and throughout the range but would start to falter beyond its 5000rpm peak.
Those examples are extreme cases, but they illustrate the differences in powerbands.
Kawasaki has decided to get more power from the ZX12 by designing the engine to operate at higher rpm. Suzuki on the other hand designed the Busa with a lower, broader powerband.The end result is that the zx12 may make more peak power than a stock Busa simply by running at higher rpm. That increased power will come at the expense of low end and mid-range power, though, where the Busa will rule. High rpm from a smaller engine with less weight generally works best on the track, while a larger, heavier, slower turning engine often makes a better, easier to ride street engine. There's no free lunch.
