How big was the VM's virtual disk to begin with? 3GB sounds like a lot, but if the virtual disk started out at 3GB and doubled, that's one thing; if it started out at 60GB and is now 63GB, that's less of a jump, relatively speaking.
Next, did the VM's size increase just from upgrading to P7, and without ever opening the VM?
Or did the VM increase in size after upgrading to P7, and then opening the VM and installing the updated Parallels Tools (and possibly doing other things too)?
If the former, no idea. If the latter, disk activity can sometimes lead to quite substantial bloat in Windows filesystems. In either case, try compressing the virtual disk and see if that helps any.
HTH,