The core of it is this: Cylinders does not necessarily equal more power; it usually equals more displacement. Displacement does not equal more power on its own.
You can have a six cylinder with more power than a V8. And that was before a few big technologies went mainstream: Turbocharging and direct injection.
Now you have Ford shipping a V6 as their main truck engine in their F150 because it is turbocharged and direct injected. Both of these technologies allow smaller engines to produce more horsepower and torque without the need for additional displacement and cylinders.
Ford's V6 with these two technologies is not only more powerful than a lot of current-generation V8s, but it is way more powerful than a lot of V8s used to be.
Even before these fancy new technologies, I used to drive an Acura with 200+ hp from a 2.0 liter 4 cylinder engine.
The amount of cylinders is pretty good short-hand for how much gas an engine will roughly use, but it's a less useful guide to how powerful an engine is anymore.
And with more and more cars gaining electric motors to augment gas engines, things are changing even more.