Do you think it might be possible that today's web is many orders of magnitude larger than the Yahoo directory?

Size of the web does not matter, it is more about not obsessing about cataloging the entire web and focusing on the content offered through the portal. I think the biggest issue to overcome with doing such a thing in 2025 is the death of links pages and webrings, they allowed the portal to give you access to web beyond the sites listed on the portal. But the blog killed the homepage and with it went their links page and the webrings they were members of.

The portals offered a very naturally curated web, the portal curated the sites it listed and each site offered a curated web as well through those links pages and webrings.

Yes, and it is mostly trash.