First, it is important to understand what affects the quality and file size of videos. Even small things such as the depth of color in your video can be important here, as the higher the color depth and the more accurate color is in your video the higher the file size will be.

The file size is also affected by the resolution (frame size), the frame rate and the codec choosen to compress the video for use on the Web.

The Center for Instructional Technology at Duke University has a great little tutorial called Tutorial: Sound and Video for the Web and Blackboard, that explains this in a little more detail.