Re-encodes an input file as an MP4 with a constant rate factor of 24
ffmpeg -i input.mov -crf 24 output.mp4
Lower crf value means higher quality and larger file size. 24 gives reasonable results with good compression.
Reduce gradually if better quality is needed.
More info: https://unix.stackexchange.com/questions/28803/how-can-i-reduce-a-videos-size-with-ffmpeg