How is this possible? (Coppied ITB of data on a 32GB USB)

Hello, I have a 1TB hard disk which I wanted to clone. This only had around 20GB of data used. So I did the following;

Coected a USB stick
Mounted this to /mnt/usb
Use dd command to copy /dev/sdb to /mnt/usb/sdb.img
Left it ruing over night

In the moing when I did ls -ah I see that in /mnt/usb, sdb.img is 1TB!

How is that possible!??

dd command I used is as follows:

root@data:/mnt/usb# dd conv=sparse if=/dev/sdb of=/mnt/usb/sdb.img bs=1024
976762584+0 records in
976762584+0 records out
1000204886016 bytes (1.0 TB) copied, 7449.56 s, 134 MB/s
root@data:/mnt/usb# ls -lh
total 16G
drwx------ 2 root root  16K Jun 30 19:50 lost+found
-rw-r--r-- 1 root root 932G Jul  1 04:16 sdb.img