The design of the engine will determine at what rpm horsepower and torque will reach their maximums. A long stroke and small bore will usually make for a lower rpm engine that develops good low speed torque. A short stroke and large bore engine is more suitable for high rpm horsepower. And of course there is a middle ground where most of us live. Then camshaft, manifolds, airflow, valves and all of the other engineering marvels seek to maximize the characteristics the engineers want for that bikes weight and gearing. These things determine at what full throttle engine speeds torque and horsepower will peak out.