MP3 files are Data, not sound patterns (like on normal CD's, from the shop). They use a lossless compression (like .ZIP files) to make the file size much smaller.
A lossless compression works by noting the Patterns in the Data, and making a note where, and when they appear. It then reads its own code (in this case, MPEG format) and unpacks the Data to a temporary storage space.
If you have ever tried Zipping up a MP3 file, you will find that it barely makes any change to the file size, because they both use the same sort of lossless compression. Lossless compression means that it has a smaller compression, without losing any quality of the original file.
Hope this helps you, and some other people.
Would you expect all that from a 13 year old? Lol