@Rosemary Circles within circles. You need one intake and one exhaust valve to have a 4 stroke engine in the first place, and they're up above the piston in the cylinder head.

And depending on valve placement, port size, how long the valve is Open and how Open it gets, you get different performance characteristics.

Small, single valves with narrow intake tubes and short cam profiles will make an engine develop maximum torque low in the rev range, and develop better peak torque. Great for Truck Engines.

Going to Double overhead cams, four valves per cylinder, short fat intake tubes, and long, aggressive cam profiles will make an engine rev a lot higher, faster, and it will develop much better peak horsepower.

It's a trade-off in most cases. This can be overcome with some tomfoolery like different length intake tubes for multiple intake valves, variable cam technology like V-tec, or the secondaries that open up like on the taurus SHO's engine.

In Honda's case, they wanted maximum revs, torque be damned. So they tried making pistons that had more longitudinal surface area to accept more circular valves in the cylinder head.

Because when you're trying to get a fluid to move through a tube, the cross-sectional Area goes up Fast compared to the Radius as you increase the tube's size. But you can only fit so many circles, in a line, inside a Bigger circle.

So cheat. Make it an oval like Honda did.