OK, in simple terms: grain reducing developers usually work by a solvent action - they dissolve away some of the silver, which by definition reduces the grain size. But by the same token, the net effect is that the effect of the original exposure is reduced, which means a loss of film speed.
From memory, if you go to most of the usual suspects to buy developers, they divide them according to their characteristics, so you can see which developers do what.
Given that I'm a large format photographer (so grain doesn't matter) photographing mainly stationary subjects (so film speed doesn't matter) I don't worry overmuch about these things for my own work.
If you want to go further into developer characteristics, read "The Film Developing Cookbook" by Anchel and Troop (check name spelling); and if you have access to a decent library, you might be able to find "Modern Photographic Processing" by Grant Haist where vol 1 has some very informative photomicrographs of developed film.
I use Rodinal which is an acutance developer (think sharpening in Photoshop/scanning terms) which increases grain (think adds artefacts in Photoshop/scanning); I find it fine on roll film and large format, but wouldn't like to try it on 35mm.