There are different zoom ratios. Fit, Fill, 1:16-1:2, 1:1 - 11:1.
Which one is LR set to. Whichever is selected last will be the default when zooming out then back in. Even if it is set to 1:16 Clicking on the image or pressing the Z key will take you from Fit to 1:16 which is much smaller that Fit.
You can set that with the Zoom slider in the Tool Bar area or in the Navigator section of the left hand side panel.
When using a Smart Preview version of your photo, available resolution on screen is reduced I believe. Might that be it?
Thanks you for this kind answer. But It doesn't have anything to do with the magnification ratio, it happens in al different ratio's. LR doesn't resolve to the maximum details when it zooms in from 1:1 to whatever magnification. For the record I'm a experienced LR user since LR 2.
I give you another example. But now LR 7.3 in 1:2 zoomed ration next the 1:2 ratio in Apple preview from the exported photo (JPG).
I'm not using smart previews, so that isn't the case.