Recently another user told me about their Home directory shrinking by 3 to 4 GB of space.  I went to investigate, using disk usage in terminal $ du -h > debug.txt showed me the problem file was; /home/(user id here)/.cache/lxsession/LXDE/run.log and it was 3.5 GB and growing.  After restart it would drop to a few megabytes in size and start growing again.  

If you find this to be annoying, a simple fix is to open terminal at the location of the run.log and delete it.  Then create a symbolic link;

$ ln -s /dev/null run.log

This will send the written data to the null device and the file will remain about 10 bytes in size.  I don't know what caused this to become that large, but most of the time (on my system)  it never exceeds even 1 GB.  None of the files placed in the ~/.cache are deemed critical, so this is pretty safe to do. If you experience issues, just remove the symbolic link and the run.log will create itself again in good time.

A typical run.log is full of messages and warnings.  Since the user was concerned with drive space, I didn't see a need to concern myself with reading several volumes of messages. I think the primary cause was from leaving the PC running 24/7 without reboot.