Non active window very large when reopened

The issue is all within photoshop. I move from one image to another in the tabs across the top, return to first image later to find it not as I left it, but massive so have to zoom out a lot to get it to fit the screen again.

Hi, could you provide screenshots?

it is not clear wether windows goes to sleep, locks to the screensaver, or even when simply opening another app ? Does Photoshop get minimized? Do you know the Ctrl+0 shortcut or the double click on the zoom tool (hold shift to affect all documents)