Getting back to the gist of your question:
Not the category names themselves - i'm referring to the amounts of sites within the existing categories - it hasn't changed
There are enormous amounts of editing that can take place without any change in the number of sites listed. A few examples:
-- an editor goes in to a cat with 1,000 sites in the slush heap of unreviewed, and spend the better part of a month cleaning out 700 duplicates/spams/deeplinks. A massively valuable contribution by an editor who is willing to do the scut work for which there is no fame and glory, and neither the public nor the next editor will see the effort.
-- an editor goes in to a cat with several hundred listings and works exclusively on improving the titles and descriptions, validating that there has not been a content change on the site that would either invalidate the listing or require a complete rewrite. At the end of the effort, the number of sites is roughly unchanged, but the directory quality is much higher.
-- an editor goes into a top level category and spends a lot of time identifying obviously missubmitted sites and moving them to the correct categories. This is a significant problem for us, caused in small part by honest people who honestly don't understand how to submit, but mostly by greedy webmasters and spammers looking for an unfair advantage, and by lazy ISPs who contractually commit to submit the sites they build (in 30 minutes using templates) -- and meet that submission commitment by dumping them in to the directory in a seemingly random manner (usually to the highest possible category because they are too lazy or too incompetant to drill down to the right category).
So those are but three examples of where significant work can be done within a given category without the public side ever showing that anything was touched.