Alright, here we go for the explanation of another test:
I've got 2 files: Michael Jackson.mp3 (size = 1488kb) Michael Jackson.jpg (size = 198kb) I put these 2 files in 1 folder named 'Test'. Then I browse with 'drives and devices' to the folder 'Test'. In the field 'Image file' automatically appears the name 'Michael Jackson.jpg'. Then I do 'Cover art ---> remove cover art'. (And answer the question "Would you like to copy this image to the cover art location you have specified in options?" with "No".)
Next: "Library Tools --> Update Library (from tags)". But then appears in the field 'image file' again the name 'Michael Jackson.jpg'. I get the same result if I do the following actions: 'Cover art ---> remove cover art'. (the question "Would you like to copy this image to the cover art location you have specified in options?" I answer with "No". Then: "Library Tools --> Update Tags (from Library) and next: "Library Tools --> Update Library (from tags)". And again in the field 'image file' appears the name 'Michael Jackson.jpg'.
As for file sizes, as you requested, the next test:
In 'Tools --> File location' I mark 'also store images in the file's tag'
Via 'Cover art --> Quick Find In File/Cover Art directory' MC searches for the file "Michael Jackson.jpg" and the result is that in the field 'image file' states: 'Inside File' (as it's supposed to be)
Size of "Michael Jackson.mp3" has gone from 1488 kb naar 1686kb. (Which is the filesize of Michael Jackson.jpg, being 198kb) So far so good.
Via 'Cover art --> Remove cover art' (the question "Do you also want to permanently delete the cover art files from your hard disk?" I answer with "No".)
Michael Jackson.mp3 remains 1686kb groot. Even if I do "Library Tools --> Update Tags (from Library)" and then "Library Tools --> Update Library (from tags)": the filesize of Michael Jackson.mp3 remains 1686 kb.
After this it is not possible to get Michael Jackson.mp3 back to its original size of 1488 kb.
I hope you can make any chocolate of my story. Bottom line is: in my opinion the (de-)tagging does not work well for cover art.