If you’re taking a similar route to YouTube, you also need a ton of CPU/GPU power and/or specialized hardware. YouTube transcodes every video into 2 (3 for videos with >~2M views) different formats in 5 different resolutions. A community-run service could skip on some of that, but it’d come at the cost of lower quality, less support for older devices, or higher bandwidth usage.
AV1 is very efficient (around twice as good as h264), but a filesize that low was almost definitely because the default encoding settings were more conservative than the ones used to encode the blu-ray. The perceptual quality of that 1.5gb file will be noticeably lower than the 44gb one