How to shrink the default storage

working in virtual box, i will get out of space to export image…
so i realized that the disk space provisiooned for the container can be reduced…

# lxc storage info default -v
  description: ""
  driver: zfs
  name: default
  space used: 5.15GB
  total space: 14.43GB
used by:
  - 3e48d0a915f2ea5b32b8a8312f91c4327e72b9f984ea0b1779b0ed3ed88530a3
  - c04c8001d7e5de9f1f94807d4c03ff11a71ac3c9d99a70370b577dbb430e1c8d
  - store
  - default

so, question: how to reduce the default storage from 14.43 to 7GB for example…??
founded a lot to extend, but nothing working to reduce…