Are you using FileVault ? If then then the diskspace will be re-used when you logoff your account. You will get an option about compressing the Filevault disk image. That could be one reason
Secondly.. 192MB is really low. Could be that you machine swaps out memory to disk. Use the terminal and do
du -k /var/vm
and you get the amount of diskspace that is used for swapping out processes to disk in KB.
Alternativly you can do
ls -l /var/vm
and manual sum up the bytes used for virtual swap files.
Even though there are som utlities that can clear out those swap files from the disk , it's not recommended since
in worst case something is in those files that an open program has a reference to. (This programs like normally
warns about this and recommends a reboot)
KInd Rgds
/Ulf