w00t.
(The reason they're so big is because they also contain the Pokéwalker. The Pokéwalker is a kind of Bandai Tamagatchi/Digivice-ish thing. More information here)
Heart Gold's box art has Ho-Oh and focuses on the Tin Tower.
Soul Silver's box art has Lugia and focuses on the Whirl Islands.
But we still don't have a release date yet... >_>