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.)
So, if you've found that your desktop Mac has lost a lot of disk space, check in /var/vm.
You can also check which mode your Mac is in by running this command:
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.

