PsCC- Re-Size to Fill Screen?

Just to add, to what he others have said, PS will generally fit the image on the screen, unless the pixel dimension at 100% makes it smaller than the screen size. I'm sure this is intended behavior, as Adobe doesn't want tiny files opened and enlarged so much that people are complaining that their images look pixelated. The shortcut cmd/ctrl-0 is quick for these situations. 

An automated approach is to use the Script Events Manager to run an Action or script to fit all images onscreen when opened.

https://prepression.blogspot.com/2021/10/photoshop-script-events-manager.html

When you say resize, do you mean Image > Image Size?

What is the exact event that makes the image smaller?

How about this key command:

If you want to go deeper with more complexity: 

https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-do-you-set-fill-screen-as-default-view-mode/m-p/8098877