Git only uses deltas in “packfiles”. Initially, each git object is written as a separate file (as you found). Later, git can pack many objects into one file, called a “pack file”. The pack file is then compressed, which automatically exploits any repetitions between the files in the packfile (or repetitions inside files).
This packing is performed by git repack. You can see it in action by invoking it manually. If you run git repack -ad on a git repo, you should see used disk space and number of files under .git/objects drop, as files are combined into packs and compressed.
In practice, you don’t usually need to run git repack. Git by default regularly runs git gc, which in turn runs git repack when necessary. So relax, git has your back :-).
The excellent “git book” also has a chapter on packfiles with more explanations:
http://git-scm.com/book/en/v2/Git-Internals-Packfiles .