Howdy,
This is an odd area where it looks like it’s a problem, but it isn’t actually a problem 
The file isn’t as large as it looks… if you run this command, you can get an accurate size:
du -hs /var/webmin/procmail.cache*
Virtualmin uses this file to store parsed email logs for faster searching. If you want to remove it, you can… but it will be re-generated when you try to search mail logs via Virtualmin.
It’s a special type of file, called a sparse dbm file. It utilizes space in an unusual manner… you can read about sparse files here:
http://en.wikipedia.org/wiki/Sparse_file
It’s completely harmless though, it’s really not using the space that it says it is
-Eric