Solvent action comes from the sulfite, but the amount of time the sulfite has to act can be as important as the concentration. Assume for a moment there was no part B in OP's developer. Reducing the amount of sulfite also reduces the pH, which, all things being equal, increases development time. This gives the sulfite more time to etch the grains. So even though you have less sulfite, the difference in graininess might be very small or even immaterial. This is the principle used in the formulation of D-25. It is D-23 with a lower pH, therefore longer development times, and more solvent action from the same concentration of sulfite.
Now add the B bath to the equation. Development is rapid in the B bath, which means the sulfite in the absorbed developer has less time to act than if full development had been carried out in bath A. This should make the difference in graininess between having 100g/L or 75g/L in bath A even smaller.
This is one of the reasons I initially asked OP how he knows TD-201 isn't sharp.
Two additional concerns: 1) A sulfite concentration of 80g/L is said to give maximum solvent effect. 2) "Sharpness" is a very complex thing and does not necessarily increase with decreased solvent activity. But that's a whole other topic.