I understand the physics behind refraction, but when added to my scene (and a fast camera move through the snow) I am surprised by the results of simply switching "refraction" on and off in the render settings.
The flakes are distributed by cloner objects through an emitter and cached with Mograph Bake. I've rebaked the motions incase this was the problem. The images below are the same frame number and contain 2 sets of cloners. The larger flakes (cloner 1) are in the same position, yet the smaller flakes (cloner 2) render at a further distance.