Most of my experimenting has been with Sodium Poly-Sialate-(mono/di)Siloxo. Perhaps these observations are true of PS resins also.
With a minimal ammount of filler the resin is very fluid. (The ammount of filler I give above is general and I have not tested that 40g is the ideal minimal ammount.) One can keep adding silica flour and the resin becomes more putty like. It stops being wet and will cure on things but not stuck to them. If instead of adding large ammounts of very fine particles, one starts adding mixed grade sand the mixture becomes thixotropic (holds its own weight but flows when vibrated).
The ammounts and particle sizes of the filler have a significant effect on the appearance or not of cracks during the drying stage. More specific experiments are necessary.
As the piece cures it is very fragile. While fluid it can be moved, and once solid it can take weight, but the slightest disturbance anytime in between these two points and the piece will crack in a dozen different places. Another symptom of this fragility is that the thixotropic mixture mentioned above will break all over if it cures while in tension, ie if it is draped over something or is hanging.
Another obstacle to having a good final product is the ammount of air bubbles trapped in the thick resin. Many of my pieces seem to be over 50% air by volume, which creates unattractive surfaces and structural weakness. No ammount of resting will de-gas the resin, nor will careful mixing. The huge and large bubbles can be vibrated out by vibrating the stirring rod. Apparently, the resin must be placed in a vacuum and the lack of atmospheric pressure lets the bubble rise.