Shrinkage is the reduction in volume that occurs as molten metal solidifies. For stainless steels and nickel alloys this is typically 2-3% by volume; for copper alloys it can be 5-6%. It must be carefully managed during casting.
How foundries compensate for shrinkage
The art of the foundry is to compensate for anticipated shrinkage when designing the mould. For simple shapes, the mould dimensions can be increased over and above the final part size to accommodate some of it.
If molten metal were poured into a mould the same size as the required component, voids could be expected. Careful design of the gates, runners and feeders that manage the flow of molten metal into the mould also restricts the effect of shrinkage.
Because these features are not part of the finished component and are removed and recycled as scrap after casting, over-compensating introduces unnecessary cost.