In general, Windows Media Video, or WMV, provides the best quality in the smallest size. Besides format, however, codecs and other aspects of video rendering make huge differences to the size.
Although WMV is known for its good quality relative to small file sizes, the choice of codec and other factors is even more significant when it comes to creating a video with a small file size. Other variables like frame rate, pixel dimension, audio sample rate, and render quality have a big impact on size. A code like DV NTSC paired with WMV can make for a small video, but custom codecs with other low variables can be even smaller.