Ok, I did some further testing myself. FYI, I have Windows 10 64-bit system.
I choose a random jpg file from the web (so others can try to duplicate this if they wish):
JPEG Image, 600 × 600 pixels
147.2 KB (150,733 bytes)
And I will import in 4 different ways into a mp3 file with MP3Tag (I tried an m4a file as well, and it gives the same results) . I read the resulting cover art file size next to the thumbnail in the left panel of the navigation window when I select the file, and give it below for each method:
(1) Rightclick and “Copy image” in Firefox, and then rightclick and “Paste cover” in MP3Tag : 346 KB
I then saved the image on my desktop (mediainfo and file explorer confirm that the 147 KB file size did not change).
(2) Open the image with the default Windows 10 photo viewer, Rightclick “Copy”, and then Paste in MP3Tag : 346 KB
(3) Open the image with another image viewer/editor (I tried GIMP and paint.net), select all, paste in MP3Tag : 346 KB
(4) In MP3Tag, rightclick and “Add cover …”, and then select the file : 147 KB
(5) Import function (I copied the image as folder.jpg) : 147 KB
So the “issue” seems to be with the copy out of a windows program, and paste into MP3Tag.
My next steps was to extract the images again from MP3Tag. The image that didn’t change, remained identical after extraction.
The image that got more than doubled in size, maintained that size when extracted, BUT the image properties have changed : the Chroma subsampling moved from 4:4:4 (in the original image) to 4:2:0. Although I am not an expert in jpg and chroma-subsampling, I understood from a quick websearch that subsampling from 4:4:4 (i.e. no subsampling) to 4:2:0 should reduce the filesize with 17% (not increase with 125% !!).
So is this a copy/paste issue with Windows 10 ?
I did another test with GIMP, with the two methods (directly opening the jpg vs. copy/pasting it into a new image), but here there was no difference in the resulting file size (and I don’t really have any ideas about other program where a file can be embedded and that I can use to test).
So my question is, does MP3Tag do any re-saving of the jpg (if done at a 100% quality, this always dramatically increases file size) when it is pasted in the cover art box?
What jpg engine/libraries does MP3Tag use?