To avoid this, I change the cover in the metadata after I have compressed it in the Editor. I use the "Set the cover for the book from the selected format" tool in the top right corner of the Edit Metadata panel. A huge un-compressed cover in the metadata will come back to haunt you not only in the Editor, but anytime the metadata is updated to the book file, like on sending to a device, updating metadata after changing comments, and so on.
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