It took me a while to figure it out, but now i’m sure. Parallels was jacking with my Macbook Air Time Machine.
If you have not idea what i’m talking about, then you probably don’t need to read this post. But just for consistency. The problem I was facing is running virtualization on my Macbook air. I use Parallels, but I assume the problem exists for VMWare as well.
The problem is that if you have your Time Machine backup device attached, and a backup is being run while any of your Virtual Machines are running, strange things happen. Usually the symptom is that even if you recently ran a backup it will seem like every backup is huge. You will also notice, if you watch the time machine status, like I do, that the amount of time before completion and the size of the backup change linearly. Making it seem like it will never finish.
What is going on is the VMs disk, and associated files are making Time Machine think there is new stuff all the time. And not only that, these files are not small as you know.
The secret I discovered, was to make sure that your VM is suspended or shut down while time machine is running. Alternatively you can selectively choose in the time machine settings not to backup your VMs, but I like everything to be backed up. Once you do one of these then you will not rapidly fill your time machine device after two or three backups, and backup time and size will be predictable and clean.