If I remember correctly, one can pour in layers, although if the resin sets quickly, you have to worry about creating voids. SmoothCast 300 has a pot life of about three minutes, and I had problems where solidified voids would form before trapped air had time to escape.
With polyurethane, the larger the part being cast, the faster it will cure, so even mixing a huge batch might be a problem since it may cure in the cup before you can pour it all in.