I have long posted that sand beds as time bombs is one of the horrible myths of modern reef keeping. A massive amount of sand can mask the need to remove phosphate for quite some time, but that is a product of the reefer not knowing their craft and not an issue with the sand bed it's self - eventually the sand cannot do any more work and that cannot be the hobbyists fault that phosphates start to rise quickly, right? :) For denitrification, the sand could fill up with that inert grey sludge that gets into sumps and stuff - I do advocate cleaning it out of sand beds every 4 years, or so, and I supposed that rinsing a bucket every few years can do the same thing.
Most people with remote DSBs just replace the sand, though.
Diffusion can be accounted for by more flow. Nobody knows if the water that enters the sandbed has 100% of the no3 removed, or 1%. Maybe less flow would yield a larger percentage drop whereas more flow would be less. In the end, the bacteria is going to multiply to handle as much as it can. I once had a large FOWLR of about 450 gallons that was loaded with huge fish that ate a ton. I had that Carib Sea Sea Floor which was larger shells and stuff since the fish would move around the sand constantly. I had a 75g fuge on this tank with just a bit of macro that grew mysid shrimp (not enough macro to do much) but it had a 4-5 inch sandbed. This smaller area kept the no3 for this whole system barelty detectable which really was a problem since the coralline grew like crazy in the display and some of these fish liked to see if my arm tasted good. In the end, surface area might matter, but my guess is that unless the surface area is saturated with bacteria, there is plenty of room for smaller areas to do more work with more concentrated populations.
