I've made what Norbert Fabritius said, but I didn't notice any compression in the existing files - df -h / before btrfs fi defragment = 658MB | df -h / after btrfs fi defragment = 658MB. New files are ok. Searching a little bring me this quote:
Running this:
# btrfs filesystem defragment ~/stuff
does not defragment the contents of the directory.
This is by design. btrfs fi defrag operates on the single filesystem object passed to it. This means that the command defragments just the metadata held by the directory object, and not the contents of the directory. If you want to defragment the contents of the directory, something like this would be more useful:
# find -xdev -type f -exec btrfs fi defrag '{}' \;
After this, my / it's occupping 656MB - nothing huge, but certainly there is compression.
Source: https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#Defragmenting_a_directory_doesn.27t_work
sudo mount -o remount /
and the new fstab options will take effect.