This could be virtually anything. If you take a 3GP file and simply concatenate 1GB of garbage data to the end of the file, you'll get a perfectly valid 3GP file that will play, but will be an entire gigabyte larger than it needs to be. The preview could be stored in the exif data and could be set to anything as well, and need not reflect any of the contents of the actual video file itself.
Point is, there's no one answer to your question. You should re-encode the MPEG as a 3GP file if having it available in that format is important to you. You can use free software like ffmpeg or HandBrake to do so fairly easily and how-tos on doing so abound.