The backup process creates a tar file and then applies gzip to it. There are two types of things in the tar file: an already gzipped sql dump and the contents of uploads (if requested.) In my case every upload file is already compressed: gz, gzip, gif, jpeg, png, zip. So the final gzipping gains only 1% of size.
I believe it would be better to demand less free space.
A previous topic from 2016 mentions disabling backup compression, but it looks like the sql dump was at that time not compressed, which shifted the tradeoffs.