Currently, when a "copy" backup occurs, the archives are copied as-is. Archive files take up a lot of space, but they also compress very well (if you deleted a bunch of tags without creating as many new ones), and since archive backups are rarely accessed, it makes sense to trade off increased access time (due to the additional work of decompression) to get a smaller file size for the archive backups. The exact method of compression (new file format, put archives in a compressed folder, etc.) will be up to AVEVA. In my testing, I used disk compression on the archive files.