HandBrake

In short, if you start with a higher resolution, higher quality source, you can get away with a bit more compression. When your dealing with lower resolution, low quality sources you typically can't be as aggressive with the quality slider. Thanks: in my mind that was the only available explanation...

In Handbrake's own documentation and on other sites I've read that smaller CRF = higher quality.

If that's true then I'm currently puzzled as to why on https://handbrake.fr/docs/en/1.3.0/work ... ality.html the recommended settings each use increasingly higher CRF values = lower quality.