EDIT: I figured out what happened. Somehow I got a jpg on my artboard and accidentally hide it. So they are not visible at all. They are duplicated every time I duplicated a artboard, so that's why my file size is so big. Still don't remember when did I got that jpg in.
Somehow I got those blue boxes on my artwork. They are only visible when moving artboard. If I just selected the artworks themselves, the blue boxes won't show up. If I duplicate the artworks, the blue boxes are duplicated too. So they make my illustrator file really big. Does anyone have any idea of how to get rid off those blue boxes? Thanks!