system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are the video files movie theaters use so much larger (100+ GB) than the files that are put on blu-ray discs which are excellent quality?

Even though the files movie theaters use are 4k, why the huge difference?
gpt
Three reasons.

Firstly, most file have a number of different versions of the film embedded with different audio (and occasionally video) sizes, allowing each screen the file is shown on to be at the highest quality. For example, a screen with an 11.1 setup will have a different audio track to a screen that uses 7.1. Projectors are rather good at downsizing video when it's in the correct aspect ratio, audio systems will always perform better when outputting directly 1:1.

The second reason is that to store 4k video, 4 times the resolution is required, which will require 4 times the bitrate to produce the same quality. The average bluray is around 30-35gb per film, thus a cinema 4k film will be 4 times that minimum (around 120-140gb) and that's before the multiple streams for audio and video.

Finally, the way the film is stored is pretty unique - each individual frame is stored as a JPEG2000 file, which, whilst each frame is stored individually in a film, it uses much better compression methods. The way that cinemas store films doesn't allow the video to look forwards and backwards to see if there's any clever way it can make things look better but take up less space. It's as close to lossless as you can get, with each frame taking up around 1mb.

Tl;dr Simply higher quality all round.