Pistons move very quickly, and faster and faster as engine speed (RPM) increases. Eventually, getting the piston to change direction becomes not only a parasitic loss of power on its own, but introduces a lot of design challenges around the strength and design of the crankshaft bearings, connecting rods, and piston retention pin. Furthermore, the crankshaft should balance the weight of the pistons, so losing weight from the pistons also loses weight from the crankshaft. Reducing the mass of the pistons often adds more headroom in these components and enables the engine to reach a higher maximum operating RPM (redline), and since horsepower is a function of torque x speed (RPM), increasing redline often directly increases peak output.

10% is massive in a piece of metal rotating so quickly.

Tiny pistons is how some motorcycles got incredible power out of small displacement(and ridiculous gearboxes). 20,000 RPM engines, 150mph/240kmh out of 250cc(4-stroke as well). And this was in the 60's.

(loud) https://youtu.be/o57JwibqCb8?t=114 https://i.imgur.com/HMvaSYh.jpg https://i.imgur.com/BefSzRs.jpg https://youtu.be/7d3hdDgFMrQ?t=5979

> metal rotating so quickly

*reciprocating

It's also why racing engines (when not limited by regulations, or when only limited on total capacity) tended to have 8, 10, 12 or more cylinders. Despite the increase in the physical size of the engine, each cylinder would be smaller (for the same displacement) allowing higher revs and more power.

Lighter pistons allow the engine to rev higher. Power = Torque * revs.

At 6000rpm a typical piston has to be accelerated from zero to ~50km/h and back to zero 200 times per second. Every bit of weight saving helps.