Discourse needs much more disk space than the size of the backup, in fact it’s the complete Discourse docker image that runs a database server, redis and much more plus all the uncompressed data.
You can check the real occupied space on your system via
df -h
Although it might be already performed by recovering space during rebuild, you can try this
apt-get autoclean
apt-get autoremove
cd /var/discourse
./launcher cleanup