What type of hardware are you using: Old laptop or computer
What YunoHost version are you running: 12.1.38
How are you able to access your server: Direct access via physical keyboard/screen
Are you in a special context or did you perform specific tweaking on your YunoHost instance ?: Nope
Describe your issue
As I discovered in my other post when you select Compress backups in the settings it seems to not do anything apart adding a different extension to the file. I’ve two backups created before and after the toggle of that setting and the web interface reports the same size, too.
Am I missing something?
Share relevant logs or error messages
Should I share the logs from a successful backup?
It does happen assuming the data being backed up actually benefits from compression.
Not sure what eats up all the space in case of Mastodon 
EDIT: I stand corrected:
$ file tuwunel-pre-upgrade1.tar.gz
tuwunel-pre-upgrade1.tar.gz: POSIX tar archive
$ gzip -l tuwunel-pre-upgrade1.tar.gz
gzip: tuwunel-pre-upgrade1.tar.gz: not in gzip format
EDIT2: On server the file is GZIPped
$ file /home/yunohost.backup/archives/tuwunel-pre-upgrade1.tar.gz
/home/yunohost.backup/archives/tuwunel-pre-upgrade1.tar.gz: gzip compressed data, was "tuwunel-pre-upgrade1.tar", last modified: Thu Jan 1 16:53:52 2026, max compression, original size modulo 2^32 439265280
$ gzip -l /home/yunohost.backup/archives/tuwunel-pre-upgrade1.tar.gz
compressed uncompressed ratio uncompressed_name
284343592 439265280 35.3% /home/yunohost.backup/archives/tuwunel-pre-upgrade1.tar
You’re right. While the web interface shows the same size for my two backups (209GiB), the on disk size is different (194GiB vs 209GiB). It seems that it does something and it’s just not clear enough.
I wonder why the not in gzip formaterror happens, though. As stated, when you download the file it is clearly a non-compressed one. Maybe yunohost decompress it before downloading, and thus this is the reason the web interface shows the same size for both.
Thank you for your double-check!