Very simply, the designers don't want to get larger dye clouds from finer grain. It would impact sharpness massively, in fact most of the things that are done in terms of specialised couplers etc are to ensure higher sharpness intra- & inter-layer from what are essentially oily clouds (couplers) forming around the grain. Scavengers, acutance dyes etc also play roles in this.
The couplers use a form of controlled starvation to ensure that as more silver forms (overexposure), there is not a major rise in dye formation or coarser 'granularity'.
Aliasing etc is best combatted by smaller, more tightly packed grains - which is why modern C-41 films are harder to bleach & fix & need specific integrated components (in the case of the former) & a fix with thiocyanate etc for the latter.
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