Docker Engine's disk space is 20G but qemu says 64G

Hi, I found I have only 20G disk space can use for docker engine, but qemu says it have 64G. How can I solve this problem?

This is my docker info

Containers: 19
 Running: 0
 Paused: 0
 Stopped: 19
Images: 35
Server Version: 17.03.1-ce
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 307
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
Swarm: active
 NodeID: ap3uchqxfbxqgdtwera5ugkmg
 Is Manager: true
 ClusterID: pu3inarjodbbs8amh6vsmesak
 Managers: 1
 Nodes: 1
  Task History Retention Limit: 5
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
 Node Address:
 Manager Addresses:
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 4ab9917febca54791c5f071a9d1f404867857fcc
runc version: 54296cf40ad8143b62dbcaa1d90e520a2136ddfe
init version: 949e6fa
Security Options:
  Profile: default
Kernel Version: 4.9.27-moby
Operating System: Alpine Linux v3.5
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 7.787 GiB
Name: moby
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 34
 Goroutines: 134
 System Time: 2017-06-01T13:21:58.764346223Z
 EventsListeners: 2
Experimental: true
Insecure Registries:
Registry Mirrors:
Live Restore Enabled: false

And this is qemu