Docker reports probably incorrect size

[manugupt1]

Description

I was comparing image sizes with nerdctl images. Both of them report different sizes. Docker reports slightly less size in bytes.

Image 1:
ghcr.io/containerd/busybox 1.28

Docker 1146369
Nerdctl 1167360

Image 2:
minio/minio latest
Docker: 254473318
Nerdctl: 261423104

The busybox image has only 1 layer with sha on docker inspect ghcr.io/containerd/busybox:1.28
sha256:432b65032b9466b4dadcc5c7b11701e71d21c18400aae946b101ad16be62333a

So, the size should ideally be the same or more. I am unable to understand why the size is less on Docker. Nerdctl uses the following code: https://github.com/containerd/nerdctl/blob/8a9bdf30fca7410b7293dd90113fbde0b1b23c45/pkg/imgutil/imgutil.go#L404-L430 where it adds the usage size of all the diffids.

Reproduce

docker inspect <...>

Expected behavior

Expected docker size to be at least as same as nerdctl's

docker version

Client: Docker Engine - Community
 Version:           23.0.1
 API version:       1.42
 Go version:        go1.19.5
 Git commit:        a5ee5b1
 Built:             Thu Feb  9 19:49:42 2023
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          23.0.1
  API version:      1.42 (minimum version 1.12)
  Go version:       go1.19.5
  Git commit:       bc3805a
  Built:            Thu Feb  9 19:46:53 2023
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.18
  GitCommit:        2456e983eb9e37e47538f59ea18f2043c9a73640
 runc:
  Version:          1.1.4
  GitCommit:        v1.1.4-0-g5fd4c4d
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Path:     /home/manu/.docker/cli-plugins/docker-buildx
  scan: Docker Scan (Docker Inc.)
    Version:  v0.23.0
    Path:     /usr/libexec/docker/cli-plugins/docker-scan
WARNING: Plugin "/home/manu/.docker/cli-plugins/docker-compose" is not valid: failed to fetch metadata: fork/exec /home/manu/.docker/cli-plugins/docker-compose: no such file or directory

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 8
 Server Version: 23.0.1
 Storage Driver: btrfs
  Btrfs: 
 Logging Driver: json-file
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 2456e983eb9e37e47538f59ea18f2043c9a73640
 runc version: v1.1.4-0-g5fd4c4d
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.1.18-200.fc37.x86_64
 Operating System: Fedora Linux 37 (Workstation Edition)
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 31.13GiB
 Name: fedora
 ID: JOQG:NR3U:MVCI:FGKM:KFOI:Z4LD:X46B:SMA4:V7ML:YIW7:GFF5:E2KI
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Additional Info

Unfortunately, Right now; nerdctl does not show image size in bytes.