Squid eating up all my free space despite max cache size limit set

[hilfubsi]

I have setup a squid transparent proxy, my SSD is 16GB and I have set the limit for Squid's cache (under General proxy settings > show advanced > Cache size in MB) to 8500.

And yet I was bitten again: squid used up all the free space, the disk was full, and OPNsense freaked out. I was lucky this time, I was able to make some space and reboot (it got corrupt last time and I had to reinstall).

df -h shows I have 13G on /, out of which 1.8G are used by OPNsense (that's what I have after deleting squid's cache). So why is a hard limit on the cache size of 8.5G still makes it use up all the free space?

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root@router:~ # df -h
Filesystem         Size    Used   Avail Capacity  Mounted on
/dev/gpt/rootfs     13G    1.8G     11G    14%    /
devfs              1.0K    1.0K      0B   100%    /dev
tmpfs              2.8G    308K    2.8G     0%    /tmp
devfs              1.0K    1.0K      0B   100%    /var/dhcpd/dev

What am I missing? How much space does OPNsense need to live happily?