Peers don't have to seed packfiles the way we're used to for "traditional" seeding of movies or music; these packfiles actually represent transition from one commit to another (instead of "all content from beginning to now"), so they are inherently ephemeral. They don't even need to be kept on the disk, because they will be generated on the fly every time a client is interested.

The DAG-osity of git really helps here (because you only have to transfer what's really needed), and the "immutability" of git helps because if your project is popular and you update your branch, everyone will want to go from the old commit to the new commit, so everyone can share the diff between them directly.