Tools such as ls or stat report the size of a directory. Of course it is not the content size. stat -c %s /home/sergio/.config 6550
What does 6550 mean in btrfs context?
Regardless of filesystem type, the size of a directory is the sum of the sizes of the struct linux_dirent (or linux_dirent64) for the filenames of the contained files. See the manual page "man 2 getdents".