[olaulaja]

You have chroma and luma the wrong way around, chroma is color and luma is light intensity. Chroma is still the thing being subsampled.

I've never really noticed effects from chroma subsampling so my guess would be the bitrate just being better with the higher resolution. Also "full res->compress->downsample" might be more efficient than "full res->downsample->compress" since downsampling is a (really bad) form of lossy compression as well.