Here again we have one of those subjects that is endlessly debated among aquarists. In all my years of experience and research there is no clear cut answer, period.
The best answer I can come up with is this.
It's not the amount of BB, but what is the most effective use of it. To clarify, everything in a cycled tank has BB. The difference is that BB on surfaces, i.e. walls, plants, gravel, etc. only effects what water is touching it, so it has limited effects, where as the BB in a filter, i.e. HOB or sponge, has water forced over it increasing the amount of water touching and being effected by it, thus cleaning the water at a higher rate and making more effective use of what's there.
Depending on the type of filter, the pad can actually play a very minor part in the process. Example, canister filters depend on ceramic or plastic media behind the filter pad to grow and maintain the bacteria colony. Marineland filters use a bio-wheel to serve the same function. In the 35 gallon sump of my own 75 gallon tank I use lava rocks behind the filter which due to their porus surface grow a huge bacteria colony. These media maintain such a large colony that when the filter pad is replaced, there is no fluctuation in filtering capability.
Through retelling of this formula people tend to take pieces of it and condense it to, "all the BB is in the filter pad." No, it just has more water being forced over it.
Sorry, didn't mean to get so long winded!