Except of image compression and geometry compression, I also noticed that 3d-layer models are very different from kmz (via download link in 3dwh), or maybe they are no kmz at all.
When you load a very large model as kmz-file, the complete geometry appears first, and then step after step, the complete textures are loaded.
When viewing a 3d-layer model, geometry and textures load simultaneously from low detail to high detail. This is similar to progressive file compression as it is available in some graphics programs for jpg, png.
Also, there seems to be something like mip-mapping, textures of different resolution are displayed according to the distance of the viewer.
Something that made me wonder is that a complex model loads faster from 3d-layer than kmz, but has less performance than the kmz model when orbiting around it. I experienced this for http://sketchup.google.com/3dwarehouse/details?mid=4dbc22f87450a7b96f8be2f10eeff72d