Short answer: Better combustion efficiency, airflow, fuel control and lots more RPM.

HP = (Torque(lb/ft) * RPM)/5280

The original horse power was one horse lifting 1 pound 1 mile (5280 feet) in one minute

So a motorcycle example, BMW R1000rr ~1 liter displacement 205hp @ 13,000 rpm is over 200hp / liter naturally aspirated. The bike still makes ~83 lb/ft of torque at 13,000 rpm. Enough fuel and air into the engine though the injectors. Then the correct compression, ignition and power stroke. Then the whole motor is designed to be optimal for this power.

Nothing off of a race track was spinning 13,000 RPM in the 1970's..