git gc & deleted branches

Hello,

I'm trying to reclaim space from an abandoned branch (never involved in any merge) using 'git gc', but it doesn't appear to work:

   mkdir testrepo
   cd testrepo
   git init
   dd if=/dev/urandom bs=1024k count=10 of=file
   git add file
   git commit -a -m 'initial checkin'
   git checkout -b test
   dd if=/dev/urandom bs=1024k count=10 of=file
   git commit -a -m 'branch checkin'
   git checkout master
   du -s .    # returns 30960
   git branch -D test
   git gc
   du -s .    # returns 30916

Here I had expected ~20000 since the branch uses ~10000.

My config is

[gc]
         reflogExpire = 0
         reflogExpireUnreachable = 0
         rerereresolved = 0
         rerereunresolved = 0
         packrefs = 1

I also tried 'git-pack-refs --all' or 'git-pack-refs --prune' but to no avail.

What am I doing wrong?

Thanks for any hints.

Regards

Guido