This is not a problem but it made me curious. - I have taken a few of my video clips (WMV format) and added music to them with the Windows Movie Maker. Common logic would tell you that this adds bytes to the clip - but quite the opposite. The clips WITH music have about 20 to 30% less bytes than the exact same clips WITHOUT music. Would anybody have an explanation for that? And I cannot observe any loss of quality or any other change whatsoever.

whs, when you modify a video with Windows Movie Maker it will always be recompressed on output. Also, you get very limited control over the output specs, so WMM is making some choices for you. The output is being rendered at a lower bitrate (more compression) than the input video.

To really see the information on what the bitrates are, you need a tool such as MediaInfo which will tell you more about a video file than you probably want to know. But, you can look at the codecs that were used to compress each file the framerates, bitrates and so much more to see exactly why the size changes.