I am working with Lightroom 6 CC on a rather new iMac and on a macbook pro, same version of LR and same version of the operating system. Detail enlargements do work very well on the macbook, but only in parts on the iMac. If the detail ist rather small and I want to enlarge it to the original size of the picture on the iMac, I can not get the whole size of the original picture. Is someone able to explain me the reason for this and - even better - how to avoid this. Of course, I can zoom the picture I got from the detail, but it doesn't help, if the picture is part of a series with other pictures (which have the right size) I want to export. In this case it comes always as a smaller picture in the exported collection.
Thanks for help!
How much of a picture you can see depends on the size in pixels of the image, the size in pixels of your monitor, and the size in pixels of the LR panels that are at the edges of your screen.
The navigation zoom control (upper left) will control how big the picture is shown to you.
The zoom on the screen has nothing to do with the size of the exported image. The size of the exported image is controlled by the Export dialog.
What image viewer are you using to view the exported images outside of Lightroom?