I upgraded to Classic CC and the converted cataog is 1/3 the size of the original. Why is this? Has it lost 2/3 of my stuff?
It's a little vague in the release notes, but as of Lightroom Classic the catalog is compressed. So it's the same catalog, just taking up less storage space. Chances are, all your stuff is still there.
(Of course, the actual images are never stored inside the catalog, so upgrading would not have deleted any images.)