In crystallization process if we use slow cooling then we can find bigger crystal than if we use quick cooling. In quick cooling process we find smaller and temporary crystal. On the other hand in a saturated solution there is little amount of impurity.
If we use quick cooling process then this non soluble impurity stick into the crystal and the crystal become defiled and impure. On the other hand if we use slow cooling then this impurity goes out of the crystal and crystals become more pure.