The cylinders will have smaller valves in the V8, but they will also be pulling in less air per individual cylinder since the cylinders are smaller so that's ok. But as a whole, the engine has a larger head and more valves, and potential for more total valve area for the 4L total displacement. That is to say the V8 will have smaller valves than the V6, but the valve area to cylinder volume should be slightly increased on the V8. In other words, maintaining a minimum useful bore size, the valve area doesn't decrease as fast as the respective cylinder volume decreases when dividing the displacement amongst more cylinders. If you DON'T take advantage of this in your design and bore/stroke, you miss one of the primary advantages of choosing the V8 over the V6 of the same displacement.
More important than the actual number and size of the valves themselves (since lift and duration can compensate for valve size) is that total head port area/volume will be greater with more cylinders for a given displacement. Even if the valve area is the same, you will have better port geometry and more head casting to work with.
It's the same principal behind 2v per cyl vs 4v or packing sand in a jar vs rocks: more smaller "things" geometrically make more effective use out of a volume/area than fewer but bigger "things". Trying to make 3 big ports for 2L (half the engine) in a short V6 head is going to run into material thickness issues with neighboring ports, etc, while filling the same 2L with a slightly longer head with 4 ports is going to result in a much better head that responds better to cam tuning and RPM.
Of course we are talking purely geometry and theory. In practice it doens't matter, you can make any engine do whatever you want with enough time and money.
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