Wasn't it supposed to save 6-7 GB over the initial Leopard install, or is Time Machine archiving the old source code *as well* as the new? - CannonGod

@tadunne from Twitter writes: "It's because every file has changed and you don't have enough room for time machine to sort it out." - Thanks, I knew it was something so conceptually simple ^_^ I'll try reformatting, though I would of thought Apple might have taken this into consideration beforehand. - CannonGod

Time Machine doesn't back up OS files so I seriously doubt 'every file has changed'. It maybe be that enough files have changed in your home directory, like your email data for example. - Paul Grav

I don't use File Vault, everything is default as when I bought the mac. Also my Time Machine had plenty of space before installing Snow Leopard and I don't download large files frequently. I mainly just surf :¬P - CannonGod

If your back up is on the same hard drive as the files you're trying to protect you're doing it wrong. >.> - pitlord

@pitlord: I have an external HDD set up specifically for Time Machine. - CannonGod