Time Machine Compressing Files???

I'm using Time Machine (for the first time) to back up my system (Mac Pro w/750Gb internal drive and a firewire 750Gb external backup). However, when looking at the contents of my external backup drive after Time Machine has finished, I notice there is sizeable difference between the amount of data on my internal and external drives.

To confuse matters more, I repeated the backup 3 separate times on the same internal volume (357Gb worth of data) and each time Time Machine yielded a different size backup; ranging from 343Gb to 350Gb. Each time I have "erased" the external drive using Disk Utility prior to running Time Machine.

After digging around I found at least some of the difference lies within the Movies_IDVD Projects folder. I have only a few projects but it appears that Time Machine randomly chose a few of the larger ones to compress?? Either that or it did not back them up all the way. These files were roughly 2-4Gb in size on my internal and only 200kb on the backup drive.


Are there other files that Time Machine is either compressing or not backing up? Should I use Time Machine at all or should I copy the files I want backed up by dragging them to the external? I would like to be able to restore my system to its current state one day if/when my intenal drive craps out (including all of my apps, etc.).

Thanks for your help.