Get a Matias Quiet Pro. It's a mechanical keyboard and it's pretty quiet. I have the Laptop Pro which uses the same keyswitches. It's so much better than a rubber dome keyboard.
I'm typing this on my favorite keyboard of all time -- the Apple Extended Keyboard II, but it would never be confused with "not bulky." Plus, you have to get an ADB-USB adapter and, at best, the keyboard will be about 20 years old. That said, they're the best keyboards ever made IMHO and, even at 20 years old, mine still types like new. Plus they're pretty quiet for a mechanical keyboard.
I also love the Model Ms from Unicomp, but those definitely aren't quiet.