I only have my bounding boxes structural. It always saved space for me. Everything else is detail. Your BSP size seems huge considering you only have 16k brushes. Do you have lots of large open volumes or something?
That is a lot of terrain type stuff in one room, maybe that's why, I dunno. I remember "outdoor" style maps always worked really poorly in the q3 engine. But I thought that was mainly a vis issue not a meta issue.. don't know what to tell you. Have you tried compiling it in a more modern q...
Well that accounts for about 4k brushes. Do you have any curved surfaces? If you use too many subdivisions on those, they can easily cost you a lot of brushes. If I had to guess though, I'd just say it's because of your one large open area with lots of non-coplanar brushes. I'm not sure if there's a...