How do I stop automatic magnification on my iMac

I reviewed the Accessibility preferences, especially the settings related to Zoom and Display, but couldn't find any helpful settings to control automatic magnification. Under the "Interacting" section of the Accessibility dialog box, the Mouse and Trackpad - Mouse Options has the Scroll setting of "with inertia," but that is related to scrolling, not magnification.

It's strange, but today I've been on my iMac quite a bit and I've only had the enlargment issue once! I'm not repeating the questionable actions as much today as I apparently have done previously, but can't determine what that action is!!!

I'm not familiar with those combos although testing them out now I see why you'd ask. No, I hadn't used those combos although I have used the combo of holding down the Ctrl key while sliding my finger on the mouse in order to zoom out and then back in. I use that method when demonstrating in class sometimes. No, this magnification occurs without my active participation, that's why it's so frustrating! Thanks for the ideas though!