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.
I searched and found a similar issue posed a long time ago.
[F] Paste Cover, V2.39P