Why has the file size increased when converting AVI -> WMV?

Question

I had some home movie footage that I saved as a 75MB AVI file back in the mid/late 90s. Today, I used Windows Live MovieMaker to make a two and a half minute clip from the original AVI. It has resulted in a WMV file that is just over 100 MB, ie. 25% bigger than the entire original clip. I was expecting it to be around 10MB given the original about 15mins.

Is this related to my choice of video format or have I done something wrong?

Answer 1

Is this related to my choice of video format […]?

Absolutely. You can encode a 10 second clip using an average bitrate of 1 MBit/s and get an 1.25 MB file. You can also encode a 100 second clip using 100 kBit/s, and get roughly the same file size. Length doesn't matter here, it's the bitrate.

The file size depends on the quality settings, the codecs involved, and lastly on the resulting bitrate. For example, a movie encoded with MPEG-4 Part II (e.g. XviD codec) is much larger than a movie encoded with MPEG-4 Part 10 (e.g. x264 codec), if you want the same quality.

You can probably try to adjust Windows Movie Maker's encoding settings to use less quality or less bitrate, but remind you that when re-encoding something that is already encoded, you will always lose quality, no matter what.

Answer 2

While WMV is a video compression format - a codec, AVI is a container format that contains codecs. It doesn't describe how the video is compressed.

However, it is likely that the codec that was used in the AVI is different to WMV, or is WMV with different compression settings.