Is there a lot of difference between blu-ray and br-rip for a movie?

I mean the blu-ray disk is about 25GB for a 720p or 1080p movie like e.g. The Dark Knight and its rip is only about 2.5GB the viewing quality and the sounds of the rip one is quite fabulous. I know there is a difference but I just want to hear opinions from experts people or who have knowledge in such things. Thanks in advance.