I am not sure if this is a bug related to Mp3Tagger or an issue with how windows handles things. I am currently Running Windows 8.1.
If I right click and copy or cut the album cover and then paste it, the resulting file size is less than the original. If I drag the album artwork to a folder and then drag the file over it is the correct file size. This has been an issue with older versions and happens with any artwork I use.
It seems that you have explored a new case, because you wrote, "the resulting file size is less than the original".
On the other hand people wrote, "the resulting file size is greater than the original".
Read there ... Cover Art & filesize
Take it as it is.