Before you produce your final video that you want to place on the web, choose the settings that will make it best for use on the web:

View tutorial on choosing from expert settings in Apple's IMovie

View Windows Movie Maker Tutorial

 

Another Strategy:

Produce the highest quality settings for the video production. This is sometimes described as "for playing on my computer" (but not "playing on the network").

Put the finished / produced file in a folder of the same name.

Now use a video converter ("transcoder") to produce the final formats that you need.