Please paste the output of "df -h" but I expect the problem to be real, you are probably just unaware of where the problem happened and of how the disk space of your USB key is used.

8Gb is not so large nowadays given that 3GB are taken by the ISO image itself.

Have you configured persistence for instance? How much RAM does your system have?

Thank you for your interest in Kali but this bug report is not actionable.
Please read https://kali.training/chapter-6/filing-a-good-bug-report/ first. We
are a small team and we expect you to provide a much more detailed bug report.

Once you have done the necessary research (possibly with the help of the
community on http://forums.kali.org or on IRC #kali-linux on irc.freenode.net),
feel free to re-open this bug with more information or to open a new report.

Hi, sorry for my delay, I was too busy. That´s my df -h output. Please tell me how to manage the space on device in order to configure persistence, which I haven´t done yet. By the way, I´m using the 2018.2 version. Thank you.

df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.5G 0 1.5G 0% /dev
tmpfs 301M 8.9M 292M 3% /run
/dev/sdb1 7.5G 3.1G 4.4G 42% /lib/live/mount/persistence/sdb1
/dev/loop0 2.5G 2.5G 0 100% /lib/live/mount/rootfs/filesystem.squashfs
tmpfs 1.5G 0 1.5G 0% /lib/live/mount/overlay
overlay 1.5G 23M 1.5G 2% /
tmpfs 1.5G 0 1.5G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 1.5G 0 1.5G 0% /sys/fs/cgroup
tmpfs 1.5G 0 1.5G 0% /tmp
tmpfs 301M 44K 301M 1% /run/user/0

Oh, and my RAM is 3 GB.

I am sorry, but this is not a support forum. This issue you are encountering is due to a lack of basic Linux skills, and not a bug in Kali Linux.

Please take time to read https://kali.training/topic/adding-persistence-to-the-live-iso/ and take advantage of the help of the community at http://forums.kali.org or on IRC #kali-linux on irc.freenode.net.