Deleting a container image does not free up the full amount of space as reported by the "SIZE" field

Issue

• When using podman or docker to remove a container image, the SIZE field reported in the output of podman image ls is not an accurate representation of how much space is reclaimed if that image is removed.
• As an example, removing a 2.65GB container image only releases 18MB of space:

$ podman image ls
REPOSITORY                                            TAG       IMAGE ID       CREATED         SIZE
<none>                                                <none>    eb15400c0cb0   9 months ago    2.65 GB

$ df -m
Filesystem                  1K-blocks   Used Available   Use%   Mounted on
[...]
/dev/mapper/rhel-root          141172 120991  20181   86% /

$ podman rmi eb15400c0cb0
Deleted: eb15400c0cb09e234c05b6ef16f2661a19079b298760e5b5f3f8c136ce0781cf

$ df -m
Filesystem                  1K-blocks   Used Available   Use%   Mounted on
[...]
/dev/mapper/rhel-root          141172 120973  20199   86% /

• What is the SIZE value representing, and why is less space than anticipated freed when the container image is removed?