Time, the forgotten factor for grain development
We always talk about sodium sulfite and low pH as contributing to "fine grain." What they really do is first, a low pH extends development time, and then the sulfite has time to work. (The sulfite also contributes to film speed, but that's another issue.)
One of the problems with diluted developers and extended development times is that grain increases.
Conversely, a quick development in a high pH developer doesn't let the grains clump, which is what really contributes to the appearance of grain. In fact, they don't really clump, but it looks that way looking through the emulsion as there is a grain here, then one a micorn below, and one below that, etc. In chaotic distribution, they look like they are clumpting.
The look of the grain will be different in a high pH developer than in say, D-76. Diafine and most two baths use a pH of about 10.5 for bath B, but it is in and out in three minutes. Generally the grain is pretty unobjectionable in Diafine and similar.