Depends on many factors.
If it is raw, uncompressed video, you compress it using a video compression algorithm (say, MPEG-4, AVC, etc.)
You can always edit it and make it shorter, or you can drop frames (say, from 60 fps to 30 fps).
If it is already compressed, then you need to "transcode it". During transcoding you can reduce the size by
a) Reduce its resolution
b) Reduce the compression bit rate
c) Reduce the frame rate
d) Use a better compression algorithm
d) all of the above