Larger formats are more forgiving for grain and a lot of other things. But grain size is more dependent on the emulsion construction than on the developer. On the other hand, the appearance of the grain is quite affected by the developer, so some scenes on larger grain film and smaller formats may be more acceptable with a finer grain developer (say D-76 or D-23) than a coarser grained one (like D-50). Some developers are confusing in this respect. Rodinal is actually a rather fine grain developer, but it celebrates the grain and makes it very apparent.
These days, sharpness may be more an issue than grain. Many traditional fine grain developers take the edge off sharpness, and smaller format negatives may (again depending on the scene) appear somewhat soft. This is mitigated to a large extent by diluting the developer, as is recommended for modern films with many of these developers. Still, there is nothing that will correct the softness produced by a PPD developer on small format film. The other side is that some developers (the Beutler formula is a prime example) emphasize edge effects. This gives the appearance of enhanced sharpness with small negatives, but it is not as apparent with larger formats. These are the so-called acutance developers. In general, most developers work well with 6X9 and larger formats, but you have to use some care with smaller formats to preserve sharpness and the adverse effects of grain.
