Why docker (with overlay2 driver storage) change the reported size of a mount point

I'm having this in my /etc/sysconfig/docker-storage

DOCKER_STORAGE_OPTIONS="--storage-driver=overlay2"

Then I mount /dev/xdvcz to /var/lib/docker/overlay2. Then I do a df -h:

# df -h /dev/xvdcz
Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme1n1    938G   72M  888G   1% /var/lib/docker/overlay2

That's correct, I have 1000G EBS device. Now I start docker with service docker start.

df -h report is now changing to:

# df -h /dev/xvdcz
Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme1n1    7.8G  7.8G     0 100% /var/lib/docker/overlay2

Okay, stop docker: service docker stop

df -h report is now changing back to:

# df -h /dev/xvdcz
Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme1n1    938G   72M  888G   1% /var/lib/docker/overlay2