I don't think it's branching. From what I understand, it's changing the file. Rather than storing a diff for non-text files, it just keeps the old versions around. So if you store a lot of binary files that change very often, it makes your repo huge.
We had this problem before we moved to a dependency manager. Our repo was almost 2gb, even though the checked out code was well under 1gb.
The way I understand it, branches in git are essentially just a pointer to a revision, which automatically gets updated as you make changes. Branches don't have any overhead.
