Questions about git-push for huge repositories

Hi all,

I meet with a strange problem:

I've two repositories, with sizes:
  - A:  6.1G
  - B:  6G

Both A & B have been 'git gc' with:
  git reflog expire --expire=now --all
  git gc --prune=now --aggressive

Since A & B share many common files, to save disk space, I'd like to merge them:
(note: branch of A & B are independent, i.e. have no common ancestor.)
   git clone --bare A  C
   (cd B; git push ../C master:master_b)

Repo C's size has grown to 12G. Doing a 'git gc' again, it drops to 6.2G.

I expect that 'git push' push only new files and commits, which will
save lots of space.
Yet it turns out I'm wrong. Since Repo A has been published, pushing branch of B
will double the repo size, which is impossible for the storage limit.

Any suggestions? Thanks in advance.

Best Regards,
Levin Du