The same principles that make your image sharper make the grain sharper, too. After all, grain is simply undesired detail on your negative. (I shouldn't use blanket statements like "undesired"; in some circumstances, grain really makes the image.)
Similarly, using undiluted or less-diluted solvent developers like XTOL and D-76/ID-11 results in less grain, but diminished image sharpness. The same principles that make grain less sharp and more invisible make your image details less sharp.