How the 10 MB target is reached
File size is bitrate multiplied by length. To land under 10 MB, the tool divides the size budget by the duration of your clip, keeps the sound at a low bitrate, and gives the rest to the picture. A short clip keeps most of its detail. A clip of several minutes is scaled down so that it still fits.
If your file is already under 10 MB, it is returned unchanged and nothing is encoded.