On regard to shrinkage ;
a ) Plastic parts do not shrink the same proportion in all directions.
b ) Shrinkage for High, medium and low impact polystyrene ( stuff most models are injected or thermo formed from ) decently molded parts in no larger than 2%.
By the way and in order to eliminate the excuse of raw materials increases for exhorbitant increases in models, the cost of above materials for each ( I should say any ) model is minimal. The cost of plastic for any model can be easily less than US$ 1.00 per pound of parts.
Best regards.