The space occupied by LVM is less than the actual size of the files. How so?

Question

Debian

There is a server with mail backups. It has an LVM volume from one disk, with a capacity of 1TB.
There are mail backups on it:
drwxr-xr-x 3 root root 4096 Oct 13 02:31 daily.0
drwxr-xr-x 3 root root 4096 Oct 12 02:31 daily.1
.....
drwxr-xr-x 3 root root 4096 Sep 22 02:33 weekly.2
drwxr-xr-x 3 root root 4096 Sep 15 02:35 weekly.3

If folders are stupidly copied to another partition, the total volume exceeds 700Gigs ... and even much more.
Although...
/dev/mapper/vmail--backup-lv1 916G 422G 448G 49% /vmail-backup

How so?
If you look at the volume occupied by individual folders, it turns out that they "weigh" 200+ gigs each .. at least the first 4 for sure, which already exceeds the 400 gigs shown above.

How so? The files in the first four folders are the same (at first glance).

Answer 1

Files are sparse (leaky). Simple example:
$ dd if=/dev/zero of=11g-file bs=1024k count=1 seek=10240
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.002087 s, 502 MB/s
$ ls -lh 11g-file
-rw-rw-r-- 1 voleg voleg 11G Oct 13 20:45 11g-file
$ du -hs 11g-file
1.0M 11g-file