If you have synced the video from your phone to your Photos app, click here.
If the video is a file stored on your harddisk, the easiest way to make it smaller is by exporting it from QuickTime (see screenshots below).
If you really have a very slow connection, it can be worthwhile to compress the video using the application HandBrake, which will result in a very small filesize without loss of quality. Full instructions about Handbrakeare here.
- or -
If the video is just a file saved to your disk, click here.
Now choose File -> Export As -> 720p
In Photos (nl: Foto's), select the video from the gallery:
Store it somewhere on your disk.
Converting will take a few minutes and some power, so you may want to connect you computer to a power source.