macOS Photos Library size is 700MB but I don't see any picture running Photos? The Photos Library seems to be a directory, but I cannot open it in Finder. I can look into it from terminal bash nd and I see several thumbnails. However, if I open Photos, there are no photos...
Actually, I have also an iPhoto Library, almost of the same size.
But if I click on it I get the error:

Cannot open migrated library. The content of the iPhoto library “iPhoto Library” has already been migrated to Photos.  Open Photos to view your library or use a compatible version of iPhoto to open this iPhoto library.

I'm very very confused, I'd like to recover the photo and/or the wasted space. Could something have happened when I upgraded my Mac to MacOS Catalina?
If you upgraded to macOS Catalina it does a number of conversions during the install and then during the first time some apps are used. In your case that would be the old (iPhoto) .photolibrary would be converted to the different (but similar) Photos Library.photolibrary format.
And yes you can't just open a .photolibrary file in the Finder and yes it is a directory that macOS treats as a file. Apps are the same kind of thing, a directory (or folder) that contains all of the code, images, configurations and other things that an app needs to run.
A .photolibrary can be opened in the Finder. To do so either Ctrl - click on the .photoslibrary file (in your Pictures folder) or, if you have your trackpad set up to right click or have a two button mouse, right-click on the file (called a &quot;package file&quot;) and select &quot;Show Package Contents&quot; from the menu that pops-up.
Inside is a bunch more folders that the photos.app uses to keep track of all the pictures it catalogs. But if you look in the &quot;originals&quot; folder your pictures will be there, but with names that only photos.app understands. So you may have to go through any and all photos there to find the one(s) you need.
And if you look for a &quot;Recently Deleted&quot; item in the Photos that will contain any photos that have been deleted in the last 30 days. So if a picture was deleted more than 30 days ago, it's gone.
And if you want to recover all the space that the photoslibrary takes up you can just delete it. When Photos.app launches again it will create a new, empty, library.
Does that answer your question?