In my upstart folder in ~/.cache/upstart
:
-rw-r----- 1 bussiere bussiere 18G août 5 17:59 gnome-session-Unity.log
The log goes mad I've tried purge logrotate and reinstall it but nope.
You should really try to learn why gnome-session is outputting so many logs, but in the meantime, you can do this:
rm ~/.cache/upstart/gnome-session-Unity.log
ln -s /dev/null ~/.cache/upstart/gnome-session-Unity.log
In my case, this problem happens when I use my Ubuntu (14.04) as a remote server and do not use screen for a long time. So I turn off X graphics (Unity) service by: sudo service lightdm stop and then do as above (delete gnome-session-Unity.log and take it to /dev/null). If I do not turn off then gnome-session-Unity.log re-appears every time. Hope this help!