I'm using ffmpeg on the command line to encode video from a QuickTime ProRes 4444p DCI-resolution source to h.265, y420p. It's working just fine, but I noticed something very odd: If I change nothing but the preset, the file size goes down as the encoding speed goes up:
Preset and resulting file size:
veryfast: 5108824 (file size starts going up from this point)
My question is: This is rather the opposite from what I'd expect. Am I missing something obvious?