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
Plugins:
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
Swarm: active
 NodeID: ap3uchqxfbxqgdtwera5ugkmg
 Is Manager: true
 ClusterID: pu3inarjodbbs8amh6vsmesak
 Managers: 1
 Nodes: 1
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
 Node Address: 192.168.65.2
 Manager Addresses:
  192.168.65.2:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 4ab9917febca54791c5f071a9d1f404867857fcc
runc version: 54296cf40ad8143b62dbcaa1d90e520a2136ddfe
init version: 949e6fa
Security Options:
 seccomp
  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
ID: G6JR:QFLC:7FC4:Y6IQ:OVXQ:NXQW:TP34:HOYE:MKLW:NRVK:KZMT:DA6U
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
Registry: https://index.docker.io/v1/
Experimental: true
Insecure Registries:
 127.0.0.0/8
Registry Mirrors:
 https://u1o24iwx.mirror.aliyuncs.com
Live Restore Enabled: false

And this is qemu