git gc don't work properly

gistfile1.txt

duke@webmaker ~/tmp/c $ git --version 
git version 1.8.2
duke@webmaker ~/tmp/c $ ll
total 80K
drwxr-xr-x  3 duke duke 4.0K Apr  4 17:13 .
drwxrwxrwt 23 root root  64K Apr  4 17:17 ..
-rw-r--r--  1 duke duke    3 Apr  4 16:43 a
drwxr-xr-x  7 duke duke 4.0K Apr  4 17:20 .git
duke@webmaker ~/tmp/c $ rm -r .git/
rm: remove all arguments recursively? y
duke@webmaker ~/tmp/c $ git init
Initialized empty Git repository in /tmp/c/.git/
duke@webmaker ~/tmp/c $ cd ..
duke@webmaker ~/tmp $ du1 c
96K	c/.git
104K	c
duke@webmaker ~/tmp $ git init gittest
Initialized empty Git repository in /tmp/gittest/.git/
duke@webmaker ~/tmp $ cd gittest/
duke@webmaker ~/tmp/gittest $ echo some >t
duke@webmaker ~/tmp/gittest $ git add .
duke@webmaker ~/tmp/gittest $ git ci -m "Init"
[master (root-commit) d444b13] Init
 1 file changed, 1 insertion(+)
 create mode 100644 t
duke@webmaker ~/tmp/gittest $ cp ~/dwhelper/Katy\ Perry\ -\ E.T.\ \(Audio\).flv .
duke@webmaker ~/tmp/gittest $ ll
total 7.8M
drwxr-xr-x  3 duke duke 4.0K Apr  4 17:27 .
drwxrwxrwt 24 root root  64K Apr  4 17:26 ..
drwxr-xr-x  8 duke duke 4.0K Apr  4 17:27 .git
-rw-r--r--  1 duke duke 7.7M Apr  4 17:27 Katy Perry - E.T. (Audio).flv
-rw-r--r--  1 duke duke    5 Apr  4 17:26 t
duke@webmaker ~/tmp/gittest $ git co -b br
Switched to a new branch 'br'
duke@webmaker ~/tmp/gittest $ du -sh
7.8M	.
duke@webmaker ~/tmp/gittest $ du --max-depth=1 -h
160K	./.git
7.8M	.
duke@webmaker ~/tmp/gittest $ git add .
gduke@webmaker ~/tmp/gittest $ git ci -m "Add some big file"
[br 8572492] Add some big file
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 Katy Perry - E.T. (Audio).flv
duke@webmaker ~/tmp/gittest $ du --max-depth=1 -h
7.7M	./.git
16M	.
duke@webmaker ~/tmp/gittest $ git br
* br
  master
duke@webmaker ~/tmp/gittest $ git co master 
Switched to branch 'master'
duke@webmaker ~/tmp/gittest $ git co master 
Switched to branch 'master'
duke@webmaker ~/tmp/gittest $ git branch -D br
Deleted branch br (was 8572492).
duke@webmaker ~/tmp/gittest $ git gc --prune=now
Counting objects: 6, done.
Delta compression using up to 3 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), done.
Total 6 (delta 0), reused 0 (delta 0)
duke@webmaker ~/tmp/gittest $ du --max-depth=1 -h
7.6M	./.git
7.7M	.

history of commands.sh

  git --version 
  git init gittest
  cd gittest/
  echo some >t
  git add .
  git ci -m "Init"
  cp ~/dwhelper/Katy\ Perry\ -\ E.T.\ \(Audio\).flv .
  ll
  git co -b br
  du -sh
  du --max-depth=1 -h
  git add .
  git ci -m "Add some big file"
  du --max-depth=1 -h
  git br
  git co master 
  git branch -D br
  git gc --prune=now
  du --max-depth=1 -h

resolve

It solves by those commands:

git reflog expire --all --verbose --expire-unreachable=now
git gc --prune=now

  or

git reflog expire --all --verbose --expire-unreachable=0
git gc --prune=0