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 brush option to make that any better for your terrain, but I'd imagine a patch mesh would be more efficient at storing the terrain internally.
The Radbump_4a is based on super old code (as far as I could tell) that is less efficient and less precise at optimizing stuff. I've reduced the size of my bsp's by as much as 50% by using Rambetter's q3map2 patched with the experimental math stuff. When I decompile compiled bsps that were based on old code, I would see vertices that had like 17.001 instead of 17.000 all over, for all dimensions on many brushes. Newer q3map2's are more precise and also as a result, you also save space because you don't need to store these slightly offset coordinates. You lose the bumpy aspect, but you might save a ton of bsp space by using one of the upgraded q3map2's.
As for my suggestion, I recommend you make a backup of your .map, then remove large sections of your map one step at a time, recompile, and test to see if you've saved any space. When you find some big thing, I guess you've found your problem.
My first recommendation would be to try removing all your terrain and see if that helps. Still, there's another 10k brushes outside of your terrain, which seems like a lot (unless you've got some curved geometry).