Saving metadata to image drastically alters filesize

Hi all,
I notice when saving tag or rating data to an image that the filesize changes drastically... a lot more than I would expect.
This thread describes it perfectly, but I didn't see any clarifying information: http://digikam.1695700.n4.nabble.com/Adding-metadata-alters-reduces-image-file-size-td4712412.html

The image itself does not appear to be altered in any way, as it compares 100% alike before/after.  Can someone shed some light on what data is being lost?

Thanks!

Same happens here.  No one knows what data is being lost?