It's not just file size either. Video basically has several different things going on, where improving on one aspect tends to require compromise on the others:

• Resolution
• Frame rate
• Quality
• Bit rate (file size)
• Encoding complexity
• Decoding complexity (which affects battery life of mobile devices viewing the content)
• Robustness for dropped or corrupted data

Over time, the standards improve, but generally benefit from specialized hardware for decoding (thus making decoding complexity a bit more complicated when serving a lot of people with different hardware).

Netflix, for example, serves a small number of very large files to many, many people on demand. That means they benefit from high encoding complexity, even if it shaves off a tiny bit of file size, because spending a few extra hours on encoding a movie that's 10mb smaller is worth it if 10 million people watch that movie, as that's 100 terabytes of traffic saved.

But YouTube/Facebook and the others with a lot of user-submitted video, they're ingesting hundreds of hours of content every minute, chopping it up into like 5 different resolutions/quality levels.

Then YouTube has a shitload of processes for determining which video gets which treatment. A random upload of a kid's birthday party might get a few hundred views at most, so YouTube cares less about file size and more about saving that computational complexity up front. But if a video hits 1000 views in a few minutes, that means it's on the cusp of going viral, and it might be worth re-encoding with the high cost encodings that save space/bandwidth.

If a service doesn't scale, it won't be necessary to have that kind of complexity in the service. But those videos will load a bit slower, use a little more battery and bandwidth to watch, be more prone to skipping/distortion, etc.

Video is hard. User submitted video is harder. Especially at scale.