Thanks @stgraber for the explanation.
It is a virtual disk. I am able to grow it from 10G to 15G.
I tried the mentioned method and i use snap to install lxd. However, when i am trying to expand the pool it showed an error.
user@test:~$ sudo zpool online -e default /var/snap/lxd/common/lxd/disks/default.img
cannot expand /var/snap/lxd/common/lxd/disks/default.img: no such device in pool
And, before trying the method, I check the path you mentioned (/var/snap/lxd/common/lxd/disks) and it was empty.
Assume, my zfs pool as follow
root@test:~# lxc storage show default
Appreciate your advice.