Atime doesn't cause a write on the filesystem level, but on the device level, and it can be quite a bit less than 1 (n reads cause m writes, where m is at last 1 and at most n). It isn't really relevant to Tarsnap.
It is easy to construct a find command with a bit of postprocessing to reveal the directories with the most new files. Find, xargs ls -s if you want size rather than file count, awk, done. Sent
