SBS 2008 disk space usage expands to fill available space

I am running SBS 2008 with 3-users on Vmware ESXI.    I am seeing a behavior which I can only describe as disk space usage on c:\ seems to expand to almost fill available space?  

A few months ago I noticed that available space on c:\ was < 8 Gigs.   Because I'm running on vmware, I was able to provision additional storage from a datastore and use server 08's normal disk management tools to expand the disk by 10 gigs.   After doing so, I located a few very large log files and deleted them and ran WSUS cleanup.

A month later, I checked disk space again.  Again, it had shrunk to < 8.   I repeated the process of expanding the available space by 20G.  This time there were no large log files to cleanup and WSUS cleared only about 40megs.

I checked again recently and the same thing appears to have happened, available space < 10 G.  Additional gigs of space are now used and I cannot locate what is using them.

The only user data files on c:\ are in exchange and those total about 2G for all 3 users.

One additional step I took this time was to run the SBS built-in backup utility to see if it would delete application logs.   The net disk space result was that available space went down slightly...presumably because of the files created by the backup to record the backup log etc.

There are no findable files much greater than 500 megs in size and only a few at that.

The behavior seems "repeatable" enough to think that there is some single SBS process that is using the space in an "opportunistic" manner.

Any ideas on where the space might be going?

Thanks,

Ken