[Bozos of Bones]

That's called cluster loss. The thing is, you have two types of sizes; the data size and on-disc size. A file 500KB large on a dist which has a cluster assignment of 400KB(example) will take up 800KB of space. That's why you lose 7GB. You must have a tonne of small files, or unused folders or other small stuff, because only one file can take up a cluster, it cannot be shared.