This problem has been observed in all betas of PW. I run the Disk Compacting Tool from Parallels Tools Center, and after a while (well, about an hour) I get an error that some problem occurred during the compacting process. Then, the Mac side of the compacting process starts, and finishes, apparently claiming success. And then when I check the disk image's size, it actually GROWS to about 1.76GB for a 2GB image. It was about 200MB smaller before the process. It's bizarre and annoying. Note that I have set up a single NTFS partition within the disk, and has enabled compression on the entire disk. The OS and other specs are on my sig.
