I noticed that cover art gets bigger in size (more than twice) when I import it from Clipboard rather than tagging it directly from external sources or from a local file.
It seems to me, that the imported JPGs are far more compressed and that by importing them to clipboard and importing to MP3Tag they are blown up to a bitmap and compressed to a lesser degrade.
I had a look at the files in irfanview and made some tests with loading and saving jpgs.
Irfanview has a default compression rate of 80 % when saving jpgs. When I used this default rate the covers nearly kept their original size. When I changed this to 100 % - which blows up the original JPGs - I got nearly the size as with mp3tag.
So I think mp3tag should save the files from the clipboard with a higher compression rate to keep them in a similar size than the original. I would suggest 70 to 80 %.
In many cases it is not possible to tag directly from external sources, because the cover is not found. So I often take an external seek (ie. Album Art Downloader), and copy the found cover to the clipboard. Saving this to a local file and importing this files to mp3tag I think takes too much time.
Therefore I would appreciate it, that the clipboad behaviour would be changed.
Because I used the clipboard in many cases in the past, I think I have a lot of MP3s that have grown bigger than necessary.