Lightroom Classic catalog size is very much larger than...

There are many potential variables here, and the exact size you see is going to depend on all of the variables. And it might be surprising that two of those variables are you, and time! I’ll explain that.

I've imported the photos using the embedded and sidecar option, the same option used by PM.

By @elianoimperato

Embedded and Sidecar is my preferred preview option because it doesn’t initially generate standard or 1:1 previews for images immediately after import.

However, if you select a different image, Lightroom Classic will build its own preview for that image on demand. So if you look at different images and different folders, Lightroom Classic will “follow you around” building previews for the images you look at. And, to try and speed image-to-image display, Lightroom Classic will speculatively build thumbnails and previews for images around the image you look at, so that if you go to the previous or next image, the preview can display without delay because it’s already built.

This means the size of the previews file is not static; it can constantly change and grow even if all you do is browse different folders.

In addition, if in Preferences / General you enabled the setting “Replace embedded previews with standard previews at idle time,” then the previews file will continue to grow when Lightroom Classic is open, even if you are not doing anything in Lightroom Classic, until all previews are replaced.

That’s why I said time can be a variable. If you leave Lightroom Classic in a state where it can continue to build speculative previews or you told it to replace embedded previews at idle time, then each time you check the previews file size later, it might be a different, larger number.

So, if you are trying to compare Photo Mechanic preview sizes with Lightroom Classic, it is not just a matter of which settings are used, but also whether you are doing anything that will make Lightroom Classic change the preview file size at any time.

In the accelerated demo video below, you see how the size of the Lightroom Classic previews file changes during three phases:

• Empty folder selected (no images in view, no speculative previewing) 
• Folder with four raw images selected (Speculative previews will build until all in the folder are built) 
• Viewing all images in catalog by scrolling Grid view (Speculative previews are building, and thumbnails are built for images that enter the viewport) 

When it says 152K at the beginning, that is for a fresh previews file automatically created after deleting the previous preview file and viewing an empty folder. The preview package at that starting point contains no images, only two files: previews.db and root-pixels.db. As soon as any source is selected that contains images, Lightroom Classic starts generating the preview pyramid subfolders you see in Ian Lyons’ screen shot, and the previews folder size grows.

From this you can conclude that if you want to see a Lightroom Classic previews file size that does not change, you must be viewing an empty source or a source where all previews have been built, you must not select any sources that contain images with unbuilt previews, and you must also not enable a setting that builds previews at idle time.

And if you are evaluating the size of a previews file, you understand that the size is not just of standard or 1:1 previews, but also thumbnails and other image pyramid preview sizes, and that the exact number of those depends on how many sources you partially or fully browsed since the last time all previews were purged — this is why I said that you are a variable. And there may be more variables than that.