Good idea.
Another thing you can do is to take a look at the imported book using the Edit Book feature, instead of the Ebook-viewer -- that will let you see the HTML+CSS+embed images and stuff... assuming you feel comfortable delving into that, that is.
It could give you an idea of what is missing/disconnected/whatever.
If converting the file shrinks it that drastically, it is possible the images are not being used in the DeDRMed copy, and the conversion removes the unused files (if so the conversion log would make a note of that fact).