You need to press the return key after cropping the image so as to let it go to full screen in the develop module.
Would suggest you to share a screenshot if that does not help.
Thank you for answering.
I have worked with LR for several years (LR4 and LR5), so I know the program very well. But this is a problem that comes only on my iMac, not on my MacBook, and it is not better when I press the return-button (or click at "Done"-button or tap at the pad when the cursor are in the middle of the cropped part of the picture)
To describe the problem a little more: When I have cropped the picture and click on the "Done"-button, the part of the picture that I don't want, turns gray (which is my background) and the cropped part of the picture that I want, remains in the same size on the screen as I cropped it, that means that it does not go up to full size (as it always on my MacBook).
And when I turn off LR and reopen it, the picture still does not come up to full size.
Does it happen with all images or a few?
As it depends upon the image's dimensions and resolution, if it is good enough to allow the full screen loupe view of that image.
Would suggest you to test with the same image on both machines and confirm.
Please post a screen-shot after you have clicked on "Done".
Try disabling the GPU. Lightroom > Preferences > Performance, uncheck Use graphics processor.
If that doesn't work try resetting the preferences. See Resetting the lightroom preferences file - updated - Lightroom Forums
Also make sure that you are running the latest version, 6.3.
Thank you for answering!
I think I have in some way solved the problem:
The pictures that had this "error" was in a Collection which I had made in JPEG-format. When I go to pictures that have RAW-formats the cropping is working fine.
But I still don't fully understand why every picture that has been cropped in this Collection, are shown cropped (not full size) on the screen on my iMac (which I bought some days ago) and not on the screen on my MacBook. I think that might have something to do with the resolutions on the screens, so I will ask my iMac-support about that.