The time spent in the dev may have something to do with coarser/finer grain, but I believe most of has to with dilution. When diluting a dev like D-76, you are cutting the amount of sodium sulfite that reaches the film. Sulfite acts as a preservative for the film, as a alkaline agent which allows the dev to work, but sulfite also "eats" silver halide molecules and when diluted, there just isn't the amount of sulfite there to "eat away" the grains of silver, thus a sharper image.
Originally Posted by Marco B
This is, or course, a very simple explanation. I'm sure you can read whole books on the subject.