Thanks for your reply.
I think I had a system failure at the time. I didn't have any message, my screen was frozen or iMac was continuously restarting.
Today, after quite a while, I upgraded my iMac to MacOS Catalina (from Mojave) and everything seems to be working fine, as far as I can see.
On that Apple link I mentioned in my first message it is written (btw, it is 3 years old):
"When you migrate a photo library from iPhoto or Aperture, the Photos app creates a new library structure but doesn't duplicate your images. Instead, Photos saves disk space by creating links to the original and preview versions of your images.
When Finder reports the file size of your Photos library, it includes all your originals and previews. It may look like your remaining iPhoto or Aperture library is taking up twice the space on your hard drive, but it isn't—your images exist only in one location, even though you may have more than one photo library.
After you migrate your iPhoto or Aperture library to Photos, you might feel tempted to delete your original iPhoto or Aperture library. Because the migrated library takes little additional space, you don't need to delete the original library."
So that migrated iPhoto library seems to take up only 65 MB of disk space at the moment which seems pretty reasonable. I can try to delete it after my Time Machine backup is complete. Just in case... :)
By the way, I am really happy about this upgrade. Even though there are some incompatible apps and plug-ins, it is negligible.
