peices that can withstand the punishment of rpm's dictate the max rpm. For most engine's, the valve train is the biggest problem.
You need to burn a specific amount of air and fuel to make enough work energy to get the job done. in this case, it's to move a car. You can adjust either the engine's part's parameters like or the RPM, both burn more A/F in the same amount of time.
So smaller engine like fours and sixes need more rpm to do the same amount of work of a bigger engine with less rpm.
A 350 can injest enough air at 9600 with the correct part's to make 650 hp.
A 454 can injest enough air at 5000 with the correct part's to make 650 also.
It take roughly ten times the money to make the 350 live at 9600.