The difference in file size you are seeing is because you are only using motionless stills in your movies. No motion, and no effects. Some codecs allows for file size reduction by writing first and last frame information for a still, regardless of the length in the video sequence. Why the transition increases file size is because the codec actually has to write info for each frame in the length of the transition.
This is not going to be the case with anybody working with actual video clips, or if someone adds motion to their stills. The codec then has to write information for every frame, resulting in a larger file size. For most projects, transitions will not make any significant change in file size.
