In order to back them up, Time Machine has to copy the disk image file. If you make any change to the contents of a disk image, the entire file winds up copied to Time Machine.

Sometimes a specific disk image in OS X will automatically mount if a file on it is targeted by the system. Here are some methods for fixing this issue.

Is anyone clear on how this works? Honestly I was surprised at how difficult that was to upgrade a Win 7 Home system with an IMG file purchased through my University.