I had some swapping issues the other day on my Mac mini, so I went into /var/vm to check how many files were there. I was surprised to find a sleepimage file; this is a file that is written when a Mac goes into hibernation mode. This file was about 9 GB - or the size of my RAM (8 GB) plus a bit.
Hibernation is only supposed to be used on laptops, but in an informal query of acquaintances (read: Twitter followers), I found that several people had seen this issue after the 10.7.4 upgrade. In addition to the space used, hibernation mode means the Mac takes a lot longer to go to sleep.
This hint from 2006 shows how to turn off hibernation and save that disk space. Essentially, you run the following command:
sudo pmset -a hibernatemode 0
(I didn't run the second command, and it doesn't seem to be needed.)
pmset -g | grep hibernatemode
If the result is 0, then you're in normal sleep mode. See man pmset for more information about the different sleep modes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2012060101021939