Photos & Reducing Clutter on macOS High Sierra
This largely depends on how you have photos imported to the Photos app. Photos > Preferences has an Importing checkbox. If this checkbox was ever disabled when you imported photos, you may not have all photos in the library itself and may exist in other places on your computer.


If you are not using iCloud Photo Library, consider selecting the image or movie in Photos, and using File > Show Referenced File in Finder. This will allow you to see exactly where the file lives.

You could export all your movie files from Photos and move them to a different hard drive or cloud storage solution, or delete the movies in Photos and replace them all with the Importing checkbox selected. You should export originals via File > Export > Export Unmodified Original for x Videos.

I also recommend making sure you have good backups via Time Machine or another service. If you make a mistake when deleting files, you can recover files.