I've got a set of .tar.gz files, which are duplicity backup files (either full backups or incremental ones). I'd like to compute which directories take the most space on backups. This will most probably be a different figure to calculating which directories take the most space on a live filesystem because I need to account for how often are files changing (and therefore taking space on incremental backups) and how compressible are files.

I know that while many other archive formats store compressed files as different entities inside the archive file, .tar.gz files do not, and therefore it is impossible to get an exact amount of storage taken in the archive by a single file after compression. Are there any tools to calculate at least some estimates?