ELI5, why do certain videos of the same length take up different amount of storage?

Question

For example, i started downloading Community on Netflix, most of the episodes are 21 minutes long, but they range in size from 76 to 116mb. They’ve got nearly identical run times, and therefore similar amounts of frames and audio, so what causes the noticeable discrepancy?

In: Technology

Answer 1

Part of video compression algorithm looks at the similarities between frames. If someone is monologuing on screen against a static background, the algorithm is smart enough to know that it only needs to store the visible background elements for one frame and repeat it, rather than a new image for every single frame. The bigger episodes probably have more movement in them.