If I clone a disk to disk like this:
dd if=/dev/sda of=/dev/sdb
The second disk will contain the data of the first disk without any space issue, but when I create an image of /dev/sda, say it has capacity of 10GB, the result file will be in my case ~48GB. Now my question is why cloned disk to image has a bigger length, while cloned disk to disk having same capacity won't produce space issue?