git gc expanding packed data?

I cloned gcc's git about a week ago to work on some problems I have with gcc on minor platforms, just plain 'git clone git://gcc.gnu.org/git/gcc.git gcc' .and ran gcc fetch about daily, and 'git rebase origin' from time to time. I don't have local changes, just following and monitoring what's going on in gcc. So after a week, I thought I'd do a git gc . Then it goes very bizarre.

Before I start 'git gc', .The whole of .git was about 700MB and git/objects/pack was a bit under 600MB, with a few other directories under .git/objects at 10's of K's and a few 30000-40000K's, and the checkout was, well, the size of gcc source code. But after I started git gc, the message stays in the 'counting objects' at about 900,000 for a long time, while a lot of directories under .git/objects/ gets a bit large, and .git blows up to at least 7GB with a lot of small files under .git/objects/*/, before seeing as I will run out of disk space, I kill the whole lot and ran git clone again, since I don't have any local change and there is nothing to lose.

I am running git version 1.6.2.5 (fedora 11). Is there any reason why 'git gc' does that?