http://www.home-barista.com/tips/grind-not-dose-t11872.html#p135729
You need to be familiar with the coffee literature, otherwise your experience with other aggregates will mislead you. The distribution of coffee grind sizes is not unimodal and close to normal; it is bimodal and extremely log-normal. -- The fines are roughly 1/10th the diameter of the coarse particles, and therefore, roughly 1/1000 of the mass. They migrate to the bottom of the puck, interlock with the coarse particles and create an aggregate that controls the rate of flow. -- The extraction is the same in principal as for all coffee brewing methods. It depends on the amount of time each particle is exposed to water, and its size. Therefore, given a proper flow rate and extraction time, it is best calculated from the size of the one coarse particle which forms the mass median (the particle at which half the mass of the puck is in larger particles, half in smaller particles). Any other measure of centrality is highly misleading, since with the log normal distribution, a far greater count of small particles is needed to balance the mass of a few larger ones. Now lets brew this aggregate in our heads and see what happens. If you look towards the bottom of the cited page, you'll see a cumulative mass graph of the particle distribution. It shows -- 25%. roughly, of the mass in fines, at particle sizes from 10 to 100 microns, -- 25% of the particles in the "small coarse" sizes from 100 to 300 microns, -- 50% in the "large coarse" sizes from 300 to 1000 microns. Espresso shots vary from 20 to 35 seconds, and around 1:1 to 2:1 in water to puck volume. So from tightest to loosest of shots, coffee to water exposure will vary by a factor of 4. The fines have a volume of 1/10th to 1/100th of the coarse ones, so the 4 times variation in brewing parameters is meaningless to them. They will always over extract or maximally extract in the first few seconds of the shot, and will be a constant as far as the cup taste is concerned. The smaller coarse particles are from 1/10th to 1/3rd of the size of the larger ones. If the large coarse particles are extracting any where close to normally, these particles too will be more or less maximally extracted by the time the shot is half way through. Their extraction will vary a little but, but not much, and their contribution to the cup will be close to the same for all shots anywhere close to decent. That leaves the 50% by weight of large particles. These are the ones that are creating almost all the variation in extraction rates. If they under-extract, the brew tastes exaggerated and balanced towards sour, if they over-extract, the taste starts going flat and finally overbalanced bitter.