Even if the resin is shrinking it wont matter. If there is enough aggregates the casting wont change. Say that you filled a cup with 90% sand and 10% water to a level. If you later dry the water out the sand will still be at that very level. The small sand stones rest on each other and the water just fill the empty space.

In the resin case, it will also bond the sand stones to each other. And when the resin is shrinking it do not want to let go of the stones. So there will be tensions but i belive this is why the E/G dont expand when heated. When heated the resin will expand a litle but not enough for the tensions to be canceled, thus there will always be forces that are trying to compress the stones.