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