To some extent it's the size of the opening, which is why ditching the snorkel makes a difference. It is more to do with the pleating of cotton filters as it give a massively increased surface area exposed to the air.

A DNA (or K&N for that matter) will flow more air than a foam filter. Foam is better at dealing with fine dust particles than cotton. Foam filters need cleaning more often as the drop off in air flow is quite marked over time (but you'd expect that as they trap more fine cr@p). Cotton filters actually filter at their best after they've got a few thousand Kilometers on them and in use don't tend to loose airflow as readily as foam.

It's all a toss up between peak airflow and filtration - you can't have your cake and eat it! For road and infrequent light off roading, then go cotton. For serious off road and in dusty conditions foam is the way to go.

The other advantage of foam is you can carry a clean and oiled spare element in a zip-lock bag where ever you travel; it's light and takes up no space.

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