I've got some livestreamed footage (encoded with constant bitrate), and I've been trying to reduce the file size for long-term storage. I figured that since encoding speed is no longer a constraint for me, I could let the encoder take more time and make a smaller file.
The original footage was encoded at 12 Mbps, but when I tried reencoding it with FFmpeg (libx264, CRF 18, and preset veryslow), the result was at 17 Mbps.
My question is, how can using these settings make a file so dramatically larger than the original? As I understand it, the point of the veryslow preset is to take a long time and make the most compressed H.264 video possible (ignoring the placebo preset), but in this case I'd be far better off doing nothing! I'd understand it being slightly bigger, because it has to encode some compression artifacts, but not a 40% increase.