There’s a balance to be struck between the density of your ground beans and the method you’re using to brew your coffee. When your beans are finely ground, you increase their surface area, so the hot water has more coffee to act on, releasing more flavour. This means that coarsely ground beans need to be exposed to the hot water for longer than finely ground beans.
Different methods of brewing expose the beans to the hot water for different lengths of time, and at different pressures and speeds. Bringing the coffee particles out of the beans in this way is called ‘extraction’ and it’s a fine art. If your coffee is under-extracted, the crema will be thin with large bubbles and the body light and watery and the taste will be weak. If your coffee is over extracted, the crema will be a thin dark foam and the body will be weak, with the taste being strong and astringent. An espresso grind, for example, should be fine to very fine; while a stovetop
espresso maker needs a slightly coarser grind.
What does this mean for the home coffee maker? Whichever method you’re using, if your coffee is a little weak, try a finer grind next time; while if your coffee is too strong or bitter, try a slightly coarser grind.