Well, the background gfx in Ikaruga and Psyvariar 2 are fairly different from one another. However, I'm certain Ikaruga doesn't use streaming video, and I don't think Psy 2 does either. My bet would be on meshes, with a lot of texture re-use. If you look at the geometry in Ikaruga, most of it is fairly simple, and although the textures are high-quality, many of them are quite plain, allowing for good compression.
