Huh? I thought we are talking about the nextcloud jail which was installed as a plugin how come your jail is 300GB?
###edit###
I see now, it sounds like you have not mapped your storage into the jail but instead chosen to store your data in the jail itself or you haven't fully grasped the concept of mapping storage into a jail.
I highly suggest you read up further on how to map storage into jails before proceeding, you might shoot yourself in the foot and lose your data and I don't want to be responsible. =>
http://doc.freenas.org/9.10/jails.html#add-storage
Normally you add storage to a jail and store data in there so if you mess up a jail, you delete it and install the plugin again which in your case sounds like you cannot do as you actually have the data inside the jail.