yafaray relies in some regards heavily on correct usage of blender units.

For small scenes with small items the volumetric step size needs to be set below 0.1 - unfortunately a step size below 0.1 can not be set, since 0.1 is the lowest possible value. As a result small scenes in volumetrics appear to be grainy. The only solution would be to  increase the size of the scene, which upon usage of eg. glass "kills" other features.

Solution: enable step sizes for volumetric scatter as low as 0.001