Here's something interesting since upgrading to PD 6 - whenever Time Machine does a backup, the size is 3.63 GB and it keeps eating up more space on my Time Capsule. Running 2.66GHz i7 with 8GB of RAM. OS X 10.6.4.
Is it backing up the virtual machine .pvm files? The virtual hard disk files are at least 3 GB. Make sure the folder containing the virtual machine files is not being backed up by Time Machine. You can also use the "Do not back up virtual machine" option in Virtual Machine -> Configure -> Options -> Backup.