I am not the last word on this, but here's what can offer... if you have an image and use Break Apart to acquire pieces of it, I'm pretty sure each piece is still a representative of the whole... carrying the weight of the image file you imported. If you delete the file from the library, you delete each piece's reference to it. However, if you take that same image and drag a copy of it to the stage and then use Trace Bitmap... you then have a graphic version of the image that you can select different pieces of and use as you wish... and you can delete the image from the library without losing the pieces.
What any or all of this does in terms of file size may be something you can do a few little experiments with to find out about... I personally don't know what the deal is there, and if I needed to know I'd be trying just that.