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?