Thanks! I was able to reproduce the issue using a dual monitor setup like you described. I logged it in our bug database so a developer can take a look at it. It looks like Photoshop is getting confused when it comes to scaling the image properly to fit on screen when you're working with a Retina and non-Retina monitor. The only workaround I have at the moment is to use Command-0 to force the image to fit on screen. I realize that's not a great alternative to what you're already doing, but it's one less key press.
I also found that I got better results if I switched to tabs -- the first image would sometimes open zoomed in, but after that, they would fit on the screen. If you can fit this into your workflow you might try it.