I'd be strongly against deleting most of the random thread for small x things threads. Yes, there's lots of transient waffle (especially in original small things thread), but there are also some excellent posts whose only crime is not being easily categorised.
If there must be a cull, Tune Association and the other word games threads would be the place to start. And probably things like old classifieds, but they're not going to free much disk.
(Fair point about block size. I remember the days when install tools would ask if you were going to use a partition for a newsgroup server or similar in order to pick a smaller block size. Though presumably the SQL database abstracts all that onto a higher plane of allocation with its own minimum size...)