I just had a look at Mawes and I think I found the problem, almost every piece of bsp in that map is totaly off the grid, and many of the bsp objects are oddly shaped. The map is over 40 meg and I deleted all of the most oddly shaped pieces and it went down to 20 something meg right away. The first thing that you want to make sure of is that you have drag grid turned on, it's that little button at the bottom of the editor that looks like a grid, this should always be turned on whenever you are moving or creating bsp, although it's fine to turn if off when moving static meshes. Also you want to avoid turning the drag grid size down too far(it's the little combo box right next to the drag grid button), most maps can be done with the drag grid set to 16, if you need to go lower than 4 than you should probably find a differnt way to do what you want to do. Secondly, bsp should be keept as simple as posible, if you need to make a shape that is more complicated than a simple cube or eight sided cylinder than you should consider using a static mesh, the general rule is to only carve out your world space with bsp and fill it up with static meshes, although filling up the space with SIMPLE bsp that aligns to the grid is perfectly fine too. So keep learning maya because you are going to need it more and more.
Considering the rared size of the map, only 3+ meg, I don't think it would be worth rebuiling all of the bsp, even after I deleted several pieces of bsp the rared size only went down less than a meg even though the unrared size went down by 20 meg. I do however think that in your future maps you need to rethink the way you use bsp.
Hopefully Steven or someone else will have someting else to add, bsp has always been one of my weak points.
I hope this doesn't produce nasty visuals in the in-car camera mode.
