Take this picture for example, where two red lines go from corner to corner.
If you open that in the Photos app, the menu covers a good portion of the upper half. I don't understand the thought process behind the menu covering part of the picture itself. The menu shoult not cover the canvas!
In the old Photos Viewer you also had a button that either scaled the picture to 100 % or to fit the screen. There was also a zoom slider. All of which was removed in the new photos app. Oh, and you could use the mouse wheel to zoom. But apparently, mouse wheel's aren't a thing any more.
I don't know what they were thinking, but it must have been something along the lines of "Hey, let's take the Photo Viewer and make it really, really frustrating to use!"