[cooper12]

    ffmpeg -i foo.mp4 -c:v h264_videotoolbox -b:v 1600k foo_out.mp4

On macOS, this uses hardware acceleration to reencode a video at a lower bitrate. My macbook is from 2012, so this does make a notable difference. There's also "hevc_videotoolbox" for H.265 if your machine supports it.

[izacus]

It will also significantly tank the quality (HW encoders are horrible at quality-per-bit ratio) - libx264 at this bitrate will make a huge difference in how good the video will look.