Mootools

So why are .skp files SO much larger than say .3ds or .obj? Is it the material handling or what? I have 454kb .obj files, and 6.56kb .mtl files which converts into 9.54mb .skp file?!

This might be due to the texture files. The mtl file itself only contains references to texture files.
If you have many texture you must also take them into account for the final size of the OBJ file.

The skp file embeds the bitmap files. The advantage is that you can move the file without breaking the texture references.

If your 3d models contains many textures, this will make the final file larger than if there is no texture.
This can explain the difference between the obj+mtl file and SKP file.