Adding a cover changes the length in metadata

[MiniDisc]

Adding a cover changes the metadata label storing the length, but does not actually lengthen the contents of the file itself.

[ohrenkino]

Could you first follow these hints please:

Hints for asking Support Requests

If you need support using Mp3tag, you can help the community members that are about to support you in finding a solution or workaround to your problem, by following these simple guidelines. Be descriptive, also in what you’ve already tried Please give a short explanation what’s not working as expected or what you’re trying to achieve. It’s best to know any details about the problem at hand and especially, what steps you’ve already tried to solve it by yourself. Helpful information also includ…

Then, after you have examined the files and your observation is still valid, then please tell us which program leads you to that conclusion.

Also, see this thread:

Adding a cover may change the length

Once I add a cover into a mp3 file, Mp3tag may extend its length probably, the bigger cover file is, the longer mp3 file I will get. I'm using Mp3tag v3.02