system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is a video taken from my phone camera a couple GB but a movie which is much longer is about the same size?

I took a 9 minute video (1080p) from my phone the other day, and it turned out to about 1.38 gigs. But a downloaded movie of the same quality (1080p) is often 1.4 or 1.5 GB while retaining the same quality?
gpt
It is possible to significantly compress video, but it takes a lot of computing power, too much for your phone to do it on the fly.

You could take your video, run it through a conversion and compression process for an hour or so, and wind up with something much smaller that looks pretty much the same.