Hi mdm13,
Thank you for using the Apple Support Communities! I understand you are having an issue zooming out within Photos on your Mac. I am happy to help.
I will be using our Photos User Guide as reference: Browse and view photos in Photos on Mac - Apple Support. Feel free to use this resource in the future as well, to find answers to many How To questions.
In your case, it sounds like you are able to see the bar which allows you to zoom in and out on a photo, but it does not change the size of a photo. I have a few questions.
-Is this happening with every photo you attempt to view, or just certain ones?
-Are you editing a photo when attempting to zoom in and out?
-With Photos opened, and an individual picture selected, if you go to View in the toolbar, are you able to click Zoom Out? Or, if you use the keyboard shortcut Command (or Cmd) ⌘ and the '-' key, does the picture zoom out?
Also, try to force quit the Photos app How to force an app to quit on your Mac - Apple Support, and restart your Mac How to turn your Mac on or off - Apple Support, then test for the same behavior.
Reply back with an update on the issue and responses to the above questions, if you need continued support.
Thank you!