Fwd: Seems to be pushing more than necessary

We have a fairly large repo (~2.4GB), mainly due to binary resources (for an ios app). I know this can generally be a problem, but I have a specific question.

If I cut a branch, and edit a few (non-binary) files, and push, what should be uploaded? I assumed it was just the diff (I know whole compressed files are used, I mean the differences between my branch and where I cut it from). Is that correct?

Because when I push, it grinds to a halt at the 20% mark, and feels like it's trying to push the entire repo. If I run "git diff --stat --cached origin/foo" I see the files I would expect (i.e. just those that have changed). If I run "git format-patch origin/foo..foo" the patch files total 1.7MB, which should upload in just a few seconds, but I've had pushes take over an hour. I'm using git 2.2.2 on Mac OS X (Mavericks), and ssh (git@github.com).

Am I "doing it wrong"? Is this the expected behaviour? If not, is there anything I can do to debug it?

Any help gratefully received.

Thanks,

Graham