Unfortunately you cannot change partition sizes these are controlled by the installers.
Did you exam the home and /var/tmp directory listings and confirm there are no user files that can be deleted?
Storage clean up will only act on Junos created files.
I did have an issue in some early 15 releases on the ACX where the partition created was too small. This had an ACX specific PR assigned so may not be applicable here. The solution was to upgrade to newer versions which did have an increased partition.