Two engines, same size
Both turn a crankshaft with a piston, but watch the red power markers: the two-stroke fires every turn, the four-stroke every second turn. That is the heart of the power difference.
The four-stroke cycle
Intake, compression, power, exhaust: four strokes and two crank turns for one bang. A camshaft at half speed opens valves that seal tightly, and a separate oil sump lubricates the engine.
The two-stroke trick
The two-stroke has no valves: the piston uncovers ports in the cylinder wall. Its underside pumps fresh mixture through the sealed crankcase and transfer port, so intake and exhaust happen during the end of the power stroke instead of needing strokes of their own.
Why more power
Twice as many power strokes per revolution means nearly twice the work per turn. With no camshaft or valves to drive, there is less friction, it revs freely, and the engine is light and simple, so power-to-weight is excellent.
The price you pay
Exhaust and fresh charge share the cylinder at the same time, so some unburned fuel escapes straight out the exhaust, and oil is burned with the fuel. That is why real gains are about 1.5x, not 2x, with worse economy and emissions.
Who uses which
Two-strokes rule where light weight and punch matter: chainsaws, dirt bikes, outboards, model engines. Four-strokes dominate cars because they are cleaner, more efficient and longer-lasting, while big slow marine diesels use the two-stroke idea with proper scavenging.