Time Machine Backup Size

I've been using WD Anywhere Backup since I purchased my iMac almost two years ago. It worked flawlessly until I tried to refresh my backups recently. The program kept freezing at the "choose destination" screen. Even after uninstalling/re-installing the app I was still having the problem. I suspect this is an issue that began when I upgraded to Snow Leopard. So, as a result, I thought this was a good time to use another tool for backups.

I have a SuperDuper serial from a while back but I haven't really jived with that program. It feels too homemade or something. Besides, I'm not interested in bootable backups or clones of my drive, I just want to backup my music, pictures, and home movies. Call me crazy but...

Anyway, I fired up Time Machine last night. It's a really cool program and it's doing what I want (albeit a bit more), but it's taking up an inordinate amount of space on my external HD. My iMac has a 320GB HD. According to the "get info" on the drive I'm using 212GB. So why, after formatting my 500GB external HD is Time Machine taking up 408GB leaving only 91GB free?

I've only been using Time Machine for a day. At this rate I'll be out of space in a week!

Yes, you would be... If you have automatic backup enabled, it's going to backup your system every hour...

Do the math....

After "doing the math" on this I just couldn't buy that my Time Machine backup was over 400GB while my hard-drive sat at 200GB. Maybe if I ran my initial TM backup 6 months ago but we're talking about maybe 6hrs from initial backup to having the external drive almost full.

I re-formatted the external drive last night and performed the Time Machine backup all over again. I'm now seeing what I would expect...213GB used on the Imac, and 201GB used on my external drive with almost 300GB free.

Much better, though I can't explain what happened during my first TM backup.