Standard Preview Size in Catalog Settings

I can close this conversation saying that I've a catalog with Photo Mechanic Plus with 103791 photos (16,57 GB) and I've a catalog in LrC with 20509 photos (10 GB, of course without 1:1 previews).

This is indeed a bit complicated. The explanation is as follows: The hardware resolution of your screen is 2560 x 1600 pixels. If you would use that as your MacOS resolution settings, then you would get tiny menus and interface items, so in practice you use a setting like ‘More space’, which is 1680 pixels wide. If Lightroom would render its interface items at that resolution, then this would look pretty bad. That is because those 1680 pixels would have to be spread out over 2560 real screen pixels, meaning they would be upscaled. Everything would look soft as the result of that. So what Apple does is using a trick. It tells Lightroom that the display resolution is not 1680 pixels, but 2 x 1680 pixels = 3360 pixels. So Lightroom renders everything at 3360 pixels now, and MacOS can downscale that to 2560 real pixels. That keeps the interface items sharp. Unfortunately, Lightroom does not only use this 3360 pixels for interface items, it also (wrongly in my opinion) uses it for image display. And that is why it thinks that a preview rendered at screen size must be 3360 pixels wide in stead of the 2560 pixels hardware resolution size.

It's due to the way that the images are scaled for the retina display. There have been many discussions and complaints here and elsewhere as to whether it should happen. 

Maybe it has something to do with your monitor size (in pixels, not inches).