I installed lion 10.7.2 on my iMac in Jan 2012 and since then the capacity of my HDD has been going down without logical reasons. Since it is not a portable mac the problem could not be the backing up automatically while not connected to time machine, I applied Clean your Mac but it only gave me 4 gb more.
Searching among hidden files I found the problem:
a "private " folder that is 420GB, in it a log folder with that load of 420GB and in it a "kernel.log.0" with 376Gb, a "kernel.log.1" with 19.31 and a "kernel.log.4" with 24.12GB, among other many files.
What to do? First time with this iMac on Lion, nothing wrong using Leopard or Snow leopard before.
Those are kernel log files and they rotate regularly (kernel.log becomes kernel.log.1 etc).
They should not be so large. In fact, that's an amazing size. They are text files! 420GB of text is amazing. That's approximately 4 scientific libraries full of science books and journals (per file). with all of them together, your system is generating a library of congress or so each month :)
You probably can't open them with a normal editor because of their size. You can probably go into a terminal and do this:
sudo bash (to become root)
head -100 kernel.log
This will dump the first 100 lines of the file. I imagine something is spewing messages very rapidly. You need to figure out what that is and stop it.