Why repository grows after "git gc"? / Purpose of *.keep files?

I have noticed that after cloning a repository (via git protocol) the
repo is packed pretty tightly and takes relatively small amount of disk
space. After using it a while and running "git gc" the repo sometimes
grows 25% or something like that.

For testing purposes I deleted objects/pack/*.keep file(s) and ran "git
gc" again. The repo resulted in small again, just like after the initial
clone. I don't have disk space problems but a repo growing about 25%
after manual "git gc" seems weird. What's the purpose of these *.keep
files? They just contain text like "fetch-pack <number> on <my
hostname>".


PS. I have merged Brandon Casey's new git-gc/repack patches. In case it
    has some effect. See the "pu" branch or "git log 9e7d5019".