It's as simple as three little words: mass, inertia and material. Reducing mass reduces the inertia that tears apart an engine. Make the parts out of lighter, stronger materials and mass and inertia are reduced.
Maintain displacement, but reduce stroke and bore size by increasing number of cylinders and mass and inertia are reduced further. And to really allow rpm to be bumped up, remove the cam and followers and replace them with electrically actuated valves and, thus, the quantum jump in revs to who knows where.
