I2P is an anonymizing network, offering a simple layer that identity-sensitive applications can use to securely communicate. All data is wrapped with several layers of encryption, and the network is both distributed and dynamic, with no trusted parties.
As of right now, total_size is just the size of a current piece. This freaks out libtorrent (see arvidn/libtorrent#36). BEP09 says total_size has the same semantics as metadata_size.