Hello! Thanks for your post. As the Mod leading the Recat team, I wanted to respond with some thoughts.
As a librarian, I do understand your reasoning, however we feel that the current hierarchy is the best and easiest for both the users and the Moderators. Multiple sub-genres would become too unwieldy to maintain. Believe it or not, we actually get complaints from users who don't like the current structure and want ALL eBooks in ONE forum! If we were to have sub-forums under the SciFi/Fantasy/Horror we'd be looking at at least:Alternate History
Child in Peril
Hard Science Fiction
Sword and Sorcery
Which, as I hope you can see, if just far too involved and complicated.
In a general way, we have modelled our genre forums on library standards. Zombie/Post Apocalyptic/Dystopian etc novels fall under the genre of Sci-Fi/Fantasy/Horror, all of which are sections you will find (I hope) in your local library. It's rare that there will be sub-divisions within those sections, at least in the libraries I have visited and worked in. The same principle applies here as it does there really: you browse the "parent" genre section looking for titles or authors that you know or you think might sound like a good read in its sub genres.
So, yes, the ratio of books in one category might outweigh another but there is not much we can do about that. Don't forget we also have a Reading Recommendations & Discussions forum
that can provide you with suggestions for new authors etc.
Thanks for your feedback though, we always welcome and are glad to hear from people.