DetlevD wrote an explanation 2014 here:
In case the image file will be transported via the Windows Clipboard:
That means, at first a JPG compressed image file will be decompressed into a "Device-Independent Bitmap" (DIB), for example, a 500x500 24bit image's data size grows from compressed 25 KB to uncompressed 732 KB.Afterwards when embedding from the Windows Clipboard into the media file, the bitmap image data will be compressed into the given JPG compressed format.
However, the initial compression factor get lost.
@Florian did a change back in v2.96:
Improvements when copying cover art via clipboard
Mp3tag now also copies the original data next to an independent DIB format when copying cover art via the Windows clipboard. Previously, the independent DIB was re-encoded to a maximum quality JPEG, which was not the best option when the image was stored as, e.g., a PNG. Now, if you paste the cover to another file, the original format and data is used.
I'm not sure if this is still currently true in Mp3tag v3.22b and if there applies any automatic compression for the given picture from clipboard.