Because bio-media is simply a place for bacterial biofilm to attach, (in reality) one media is no better, or worse than another, whether its lava rock, scrubbies or some exotic maze with interstitial spaces.
There is no magic, one over the other, just a surface to live on.
The bacterial population rises or falls depending on the fish population and the waste it produces. Your bacterial population will not rise on a surface just because its bigger, or hs more holes, unless there is an appropriate amount of food.
The one caution, if you let your media get choked with debris, if those interstitial spaces get plugged, the biofilm, can get choked, smothering the aerobes, and making it less efficient.
I believe there is a major misunderstanding with many aquarists about filtration. That is to say, the belief that filters remove bad stuff from the system. This is not the case, filters (to me) are like sweeping stuff under the rug, unless those filters are often cleaned, unless one regularly purges that media of debris, all that junk is still in the tank, still in the system, and still creating problematic semi-toxic substances, like nitrate.
This is true for canisters, sumps, or any kind of filtration. If you don't get rid of gunk, it may be out of sight, but its not out of the system.
