Change Notes 2015.8
You can now zoom to fit and zoom to fill. Particularly when using ultra high-resolution (i.e. 4K and 5K) monitors, prior versions of Lightroom would not completely fill the Loupe window.
So how do I get the old behavior?
I want images bigger than my Lightroom window to be fitted and smaller ones as they are.
Do I have to go back to the old version of Lightroom??
Clicking every time with the mouse on every image(magnifying glass icon on the picture) is absolutely no solution.
How can you change the behavior and don't create an option to choose between the old and new one???
I do have a 4k monitor with 100% scaling, do you know how ridiculous 800x 600 pixel images look like with your new fitting behavior?
If you don't like the feature as it is then your only choice would be to uninstall. Then download and install a previous version. But that will mean that you will longer benefit from updates from Adobe unless you can convince them to change the feature back to what it was previously. Here is a link to the website where you can express your concerns and make feature requests.