Perform a destructive video trim in OSX Photos
The only way to reduce the file size of a Video is to change its resolution (quality).

For that you will use the Export command.

Once exported to a lower resolution, the process is not reversible.



Standard definition (480p) is good for standard-definition TV. 

If the video will play on a high-definition TV or display, choose one of the high-definition formats: 720p or 1080p. 

4K is used for ultra-high-definition TVs and displays. 

720 is most common and a good balance between good picture at reasonable file size.