Mac disk free space automatically decreases I am running Snow Leopard (10.6.8) on MacBook Pro. A few days ago I found my free disk space is too low, so I tried to clean un-necessary files form disk myself and using some programs like CleanGenius. 

There was 1.7 GB free, but after a while I found the free space is again zero. Again I cleaned up some other files and had 1GB free, but after a few hours I found the free space is again a few megabytes. 

During this time I have not installed new program, downloaded new things, or copied any files to my computer. 

This is not new to me, once before I cleaned up 7GB of my disk and after a few hours I found my free disk space is close to zero. 

Do you have any suggestion why this happens and how can I prevent it? I have up-to-date registered anti-virus.
I found my problem to be from Mail application. When it was open, the disk free space was reduced every minute. 

I just removed my Gmail account from the Mail application and about 40GB was recovered back to my free space! Also the automatic decrease of free space is stopped now.