The logical size of the volume. By default, creating a volume establishes a reservation for the same amount. Any changes to volsize are reflected in an equivalent change to the reservation. These checks are used to prevent unexpected behavior for users. A volume that contains less space than it claims is available can result in undefined behavior or data corruption, depending on how the volume is used. These effects can also occur when the volume size is changed while it is in use, particularly when you shrink the size. Extreme care should be used when adjusting the volume size.
Though not recommended, you can create a sparse volume by specifying the -s flag to zfs create -V, or by changing the reservation once the volume has been created. A sparse volume is defined as a volume where the reservation is not equal to the volume size. For a sparse volume, changes to volsize are not reflected in the reservation.
For more information about using volumes, see ZFS Volumes.