backup on volume much larger than fileset

The subject is a bit short, let me explain:

Bacula has a job defined like this:

Job {
   Name = "ivy-job-projects"
   Type = "Backup"
   Level = "Incremental"
   Messages = "Standard"
   Storage = "File"
   Pool = "File"
   Client = "ivy"
   Fileset = "ivy-projects"
   Schedule = "WeeklyCycle"
   JobDefs = "DefaultJob"
   SpoolData = no
}

With this Fileset:

Fileset {
   Name = "ivy-projects"
   Include {
     File = "/home/sgw/projects"
     Options {
       Signature = "Sha1"
     }
   }
   Exclude {
     File = 
"/home/sgw/projects/packer/my-work/packer_sgw_builds/packer_output"
     File = 
"/home/sgw/projects/docker/qemu-docker-1/qemu_storage/debian/*.iso"
     File = "/home/sgw/projects/packer/builds/cache"
   }
}

While I am quite sure that the JobDef is not the issue (another Job 
based on that works fine), here the:

JobDefs {
   Name = "DefaultJob"
   Type = "Backup"
   Level = "Incremental"
   Messages = "Standard"
   Storage = "File"
   Pool = "File"
   Client = "tx100-fd"
   Fileset = "Full Set"
   Schedule = "WeeklyCycle"
   WriteBootstrap = "/opt/bacula/working/%c.bsr"
   SpoolAttributes = yes
   SpoolData = no
   Runscript {
     RunsWhen = "After"
     RunsOnClient = no
     Console = ".bvfs_update jobid=%i"
   }
   Priority = 10
}


Now, what's the problem?

As you can see above, I try to backup the directory 
"ivy:/home/sgw/projects" minus some excluded files etc

# du -sh /home/sgw/projects
29G	/home/sgw/projects

But the resulting backup on bacula is ~ 164 GB in size!

Why that?

I browsed the directory for links etc ... nothing special.

In the details of the Fileset:

OneFs: true
Recurse: True
HardLinks: True

If I disable Hardlinks, the Estimate looks the same as before.

I browsed the Files in the Job to spot errors, with ~400.000 files 
that's a bit tough.

That directory is located in a btrfs-pool, might that play a role here?

For comparison I created a second task for the parallel folder 
"projects_legacy". This one works great, same settings etc -> correct 
size on the volume(s).

Any hints here? Thanks a lot, this would save me time and space on storage!