Sometimes it is necessary to speed up videos and/or reduce the resolution. This will typically result in a much small file size. ffmpeg and avconv allow video file manipulation on the command line.
These commands work on Debian 10, and will likely work on Ubuntu and other Linux distributions.
Speed up video, 2× speed: ffmpeg -i input.mp4 -filter:v "setpts=0.5*PTS" output.mp4
Reduce resolution down to 1280×720: avconv -i input.mp4 -s 1280x720 output-small.mp4
Note that the metadata of the video, such as the resolution, can be read with the utility: