why the nfs volume take the host disk space in kubelet folder

Question

Today I found the host kubelet(v1.21.3) path take 6GB + space, the folder path like this:

/var/lib/kubelet/pods/8aafe99f-53c1-4bec-8cb8-abd09af1448f/volumes/kubernetes.io~nfs/nfs-reddwarf-postgresql-pv-general

the folder nfs-reddwarf-postgresql-pv-general folder was in the NAS file system which was monunted to the host path /data, why still take 6GB+ spaces? what should I do to make it using the NAS spaces not the host? The host only have 40GB space. This is the disk spaces check command:

[root@k8smasterone kubernetes.io~nfs]# du -sh nfs-reddwarf-postgresql-pv-general
6.1G    nfs-reddwarf-postgresql-pv-general

this is part of the NFS mount config with PV:

spec:
  capacity:
    storage: 8Gi
  nfs:
    server: 12345-xxx.cn-shanghai.nas.aliyuncs.com
    path: /k8s/reddwarf-storage/postgresql
  accessModes:
    - ReadWriteOnce
  claimRef:
    kind: PersistentVolumeClaim
    namespace: reddwarf-storage
    name: data-reddwarf-postgresql-postgresql-general
    uid: 0bc07cdb-d673-4038-beba-171b3cb83a16
    apiVersion: v1
    resourceVersion: '21952656'
  persistentVolumeReclaimPolicy: Retain
  mountOptions:
    - vers=4.0
    - noresvport
  volumeMode: Filesystem

this is part of the PVC config:

status:
  phase: Bound
  accessModes:
    - ReadWriteOnce
  capacity:
    storage: 8Gi
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi
  volumeName: nfs-reddwarf-postgresql-pv-general
  volumeMode: Filesystem