Git packs are mostly made up of deltas. Git is remarkably efficient at diffing files. And because it's all handled at the the storage level it doesn't matter what order the files are in, it generally packs all the revisions of a file from largest to smallest with the a compression window several versions wide. The problem is that it's still a lot of data when you have significant binary assets.

Edit: The default is to delta files up to half a gigabyte in size.