Tag Copy/Paste is not equivalent to manual editing

Using an audio file with manually entered tags and cover art, if I use tag copy, then tag paste the tags to a backup of the previous file that has different tags, the updated backups will have slightly smaller files sizes, usually 10-20 bytes. Checking the extended tags, there weren't any differences, but I'm concerned about updating my backups with this.

My specific steps were:

  1. Start with an audio file and a backup audio file.
  2. Edited several field in the left panel. Saved.
  3. Removed and replaced the cover art. Saved.
  4. Right clicked tagged audio file and clicked tag copy.
  5. Right clicked backup file and clicked tag paste.
  6. Checked file sizes and the backup was smaller.

I couldn't isolate one specific step that would cause the issue. Updating only the tags, or only the cover art did not seem cause a difference in file size. Only doing all of the steps would give me a smaller size.

I could not reproduce it.
Copied a file.
Checked the size: both 415kB
Copied the tags from the source file.
Pasted the tags to the target file.
Checked the size: both 415 kB.

So: it looks to me as though you really make some substantial modifications to the files e.g. changing the cover is one of these.
But: have you have sure that there are not other tag versions in the source files that you don't display?
I refer esp. to APE tags. If you have them in the files but you don't read them, then you also don't copy them.
To check: load the source and the target file. Select the source file. Open the extended tags dialogue and check the dialogue title: does it say anything with APE?
Now switch to the target file: is the information in the dialogue title identical?

What happens if you really apply the modifications manually to the backup file?

I was thinking the same thing about the other tag versions like v1 and APE. Plus could it be related to padding as well?

Please provide an example file with detailed instructions on how to reproduce this. My ad-hoc test produced an identical file.

To the earlier responses:

  • The files I was editing were FLACs and I don't think they were using APE or v1 tags. At least, when I open the extended tags dialog, the title says "Tags FLAC (FLAC) - ..." for original and copied files.
  • I tried making the same manual edits to both files and they do came out to the same size.

I did some more testing recently and I found a way to consistently reproduce the issue for me. I got an error trying to upload my test files here, so I put them on Google Drive:
https://drive.google.com/drive/folders/1CTH57BeeoOJFSxaUddtFFoTyCB1VPXzM?usp=sharing

The exact steps I did were:

  1. Copied "Ori, Lost in the Storm.flac" to "Ori, Lost in the Storm - Copy.flac" in Windows Explorer.
  2. Started Mp3tag and added both files to the list by drag-and-drop.
  3. Selected "Ori, Lost in the Storm" and deleted everything from the Comment field in the left panel.
  4. Used Ctrl+S to save.
  5. Right-clicked the cover image and selected "Remove cover".
  6. Right-clicked the cover image area and selected "Add cover".
  7. Used the file dialog to select "cover.jpg"
  8. Used Ctrl+S to save.
  9. Right-clicked "Ori, Lost in the Storm.flac" from the list and selected "Tag Copy".
  10. Right-clicked "Ori, Lost in the Storm - Copy.flac" from the list and selected "Tag Paste".
  11. Compared the two files and found different sizes, 6,055,850 bytes for the original and 6,055,800 bytes for the copy.

A few additional things I noticed were:

  • Skiping the first save at step 4 seemed to avoid the size difference (both files were 6,055,800 bytes).
  • Saving between removing the cover and adding the cover (steps 5 & 6) also seemed to avoid the size difference (both files were 6,055,800 bytes again).
  • If I didn't restart Mp3Tag between each test attempt, I couldn't consistently reproduce the size difference.

All of this was done using Mp3tag v3.11f running on Windows 10 Pro 64 bit.

Hope this helps.

This comes down to different results in padding calculation when you use the two different methods. There is no difference in the actual metadata (incl. cover art), only a difference in reserved free space for future changes.