It is 3 times heavier than my SVN clone was.
No, it's not. There is no such thing as a "SVN clone", as it's a centralized version control system. Users only checkout the latest revision as their working copy. The git equivalent to a SVN checkout would be a shallow clone. The quivalent of a git clone in SVN would be a dump of the SVN repository.
As git is decentralized, user always clone the complete development history at first, and the merge new commits into their local branch (or rebase on top of it).
Openfires git repository is ~ 1 GiB in size. This is similar to what it takes as SVN repository, but in fact, it's less because git re-uses objects with the same SHA1, i.e. files with the same content from different commits/branches are only saved once. IIRC SVN doesn't do this.
Some voices already said that we should strip some old binaries out of the repo. I can't stress it enough that I don't think that this is necessary. Disk space is cheap those days and so is bandwith. The initial clone of Openfires git repository may take a while, but all further pulls are incremental and, if they don't contain big binaries, very fast.
What's really important is that some one steps up and migrates Openfires build system to maven/gradle so that the binaries in form of third party libraries can be removed (if possible).
If you are interested in doing this, just join the official igniterealtime MUC: open_chat@conference.igniterealtime.org