Ka, the basic grain size is manufactured into the film. You can reduce the grain size somewhat by using a silver solvent developer - at the expense of some loss of sharpness and acutance.
My developer preference for Delta 100 is Pyrocat-HD. This developer yields maximum sharpness/acutance with minimum grain and excellent tonality. I expose for the shadows and develop in Pyrocat-HD with semi-stand agitation.
With Delta 100 120 roll film I use Pyrocat-HD diluted 1 part A plus 1 part B plus 100 parts water and develop for 16 minutes, 30 seconds at 68 deg. F.
I gently agitate by inversion for the first 20 seconds, then let the tank stand. After 5 minutes of standing quietly, I agitate with 2 gentle inversions followed by a 5 minute stand, then 2 gentle inversions (at 10 minutes). I dump the tank at 16 1/2 minutes followed by a water rinse - no stop bath, followed by a non-hardening fix.