The default size of tmpmfs (tmpsize="20m") is not sufficient for pkg operation. If you do not increase the size of /tmp, "pkg update" operations will fail with the message:
pkg: archive_read_extract(extract error): No space left on device
Workaround: increase the size of the RAM disk on /tmp, or unmount the RAM disk.
Proposed solution: the default tmpsize in /etc/defaults/rc.conf should be set to a higher value.