Inspired by RedGate's upcoming SQL Instant Clone, I'm using virtual disks to store my test database and gain fast clones via differencing disks. I created a 30 GB VHD and restored a 18 GB database from a full backup using WITH NORECOVERY to be able to later restore a differential backup on top. I then created a child differencing VHD and restored a 1 GB differential backup. I expected the differencing VHD to grow to 1 GB after the restore, but instead it grew to 15 GB. Why is this?