I think of the drainage layer not as a layer of different sized particles, but of a layer that is composed entirely of material that will not easily degrade. Akadama, fir bark and all organic material will decompose over time. The drainage layer in my pots is made up entirely of pumice or lava rock, and is the same size as the soil above it. That way I can be sure that a few years down the road there is still a structure to the soil on the bottom of the pot. That said, the breakdown of components above likely clogs the structure anyway.