swf format created by FlashMX

I peeked the binary of swf created by FlashMX. It seems that the format does not conform to the swf format described for Flash5 or Flash4 ?

Does anyone know anything macromedia talked about this new swf format ?

They put an extra layer of compression on it, I think it makes the .swf almost half as large as it otherwise would be. That could be the difference.

Thank you very much.