Areas are too big, and the overall floorplan is too flat. Make use of the platform and introduce some more verticality into the level - Multiple floors, balconies, more extreme height differences.
The look is also important. I'm having trouble with this myself, but it's something i need to work on - Make every surface look 'interesting'. You don't want places in your map that look drab or unfinished. Check Hourences' levels for what i mean
A wall doesn't need to be flat and have a very plain texture. You may think it needs to be that, but it doesn't. Look at other levels in UT3 aswell, where certain walls actually look like storage areas with girders sealing them off. Stuff like that makes your level seem much larger than it actually is, without compromising gameplay by actually making it too large. It also makes it seem a lot more alive and involving, but more importantly, such details also help the player identify where he is on the map. Distinctive features help more here than just layout/floorplan! You don't want the player to get lost in such a small map, which can happen with such lighting and homogenous texturing.
One thing you could do is draw a floorplan out BEFORE you start building. This may seem irrelevant, but it gets the flow into your head quite a lot better.
After that, make the very basic BSP (with ambient lighting), to get the gameplay figured out. It's much easier to change the layout when it's not been detailed out yet.
Once the level plays well, start replacing the bsp with more detailed stuff, and rounding off the rough edges.