File smaller after saving with Mp3tag

My original MP4 audio file size is 7626KB.
After saving with Mp3tag, with no changes made to file, file size is decreased to 7184KB (loss 442KB, which is pretty big for an audio file).

Can you explain what was lost in 442KB after saving with Mp3tag?
In the future releases, could Mp3tag keep the file original (with no loss of size) after saving with it?


  • This happens with every MP4 audio file
  • With some other tag editors, MP4 file is still original (7626KB)

How was this file created?
iTunes files seem to have some big free atoms that are completely empty and Mp3tag removes them

Meet the Robinsons - Little Wonders.m4a

You can try this with every other MP4 audio file; and everytime, after saving with Mp3tag, there is a big loss in file size.

And Mp3tag shouldn't if user didn't specify it to do so.

Only empty space (padding) is removed. The files are about 5% bigger than needed.
I don't think there will be an option for that.

Again, I think there SHOULD be an option for Mp3tag to keep or delete that. Myself wanted it to be kept, and some may want to delete it.