confusion about the container size

[sailorvii]

I'm confused about the container size.
After adding some new empty files, I show the container size, it's 25B. Then I delete the file, and the container size becomes 50B.

Step 1:

~# ls -lt "/var/lib/docker/overlay2/60ec18d730cfb706433358bcd949caad9dad0ce360724f507107a84159538edf/diff"
total 4
-rw-r--r-- 1 root root    0 Apr 25 03:25 test
-rw-r--r-- 1 root root    0 Apr 25 03:25 hello
drwx------ 2 root root 4096 Apr 24 07:20 root
root@weiubuntu:~# docker ps -s
CONTAINER ID   IMAGE     COMMAND          CREATED        STATUS       PORTS     NAMES              SIZE
6ce521e7b372   busybox   "sleep 100000"   22 hours ago   Up 2 hours             adoring_dubinsky   25B (virtual 4.86MB)

Step 2:

root@weiubuntu:~# docker exec -it 6ce521e7b372 sh
/ # ls
bin    dev    etc    hello  home   lib    lib64  proc   root   sys    test   tmp    usr    var
/ # rm hello
/ # rm test
/ # exit
root@weiubuntu:~# docker ps -s
CONTAINER ID   IMAGE     COMMAND          CREATED         STATUS          PORTS     NAMES              SIZE
6ce521e7b372   busybox   "sleep 100000"   22 hours ago    Up 2 hours                adoring_dubinsky   50B (virtual 4.86MB)
root@weiubuntu:~# ls -lt "/var/lib/docker/overlay2/60ec18d730cfb706433358bcd949caad9dad0ce360724f507107a84159538edf/diff"
total 4
drwx------ 2 root root 4096 Apr 24 07:20 root

Env info:
Client:
Version: 20.10.12
API version: 1.41
Go version: go1.17.3
Git commit: 20.10.12-0ubuntu4
Built: Mon Mar 7 17:10:06 2022
OS/Arch: linux/amd64
Context: default
Experimental: true

Server:
Engine:
Version: 20.10.12
API version: 1.41 (minimum version 1.12)
Go version: go1.17.3
Git commit: 20.10.12-0ubuntu4
Built: Mon Mar 7 15:57:50 2022
OS/Arch: linux/amd64
Experimental: true
containerd:
Version: 1.5.9-0ubuntu3.1
GitCommit:
runc:
Version: 1.1.0-0ubuntu1.1
GitCommit:
docker-init:
Version: 0.19.0
GitCommit: