Shrinking is process done to reduce the size of the partition. Before imaging the disk partitions, you can shrink them to decrease the space occupied by the primary partitions and logical drives. This partition can be imaged and deployed on target computers with lesser disk space.
If your organization has computers with different hard disk sizes, you can shrink the partition and image those computers to avoid disk size incompatible errors during deployment. If partition shrinking is done before imaging, the image can be deployed to any computer irrespective of the hard disk size.
While creating an image you can shrink partitions in the OS deployment console. Follow the steps given below to shrink partitions,
Now the partitions will be shrunk to the allowed size before imaging process. To revert/extend the shrunk partition refer this document.