I know there are limitations in their design based on how capacitive screens work, but all the ones I've seen have a large, bulbous tip. I'm guessing this is to ensure enough contact is made to register, but are there any other designs that would be more precise? I find that when using the larger tips like you usually see it ends up covering up what you're using in drawing applications, making it only marginally more useful than just using your finger in the first place.
DS is resistive touch screen, which detects touch by pressure. Any modern smartphone is capacitive, which basically detects the current of the touch screen interacting with your finger. While it does allow for much smoother touchscreen operation, it means that if you press plastic against it there is no electrical current that would go through it, meaning no register of a touch.
The current solution is to have a stylus made of metal with a special ruberrized tip. The sylus basically just bridges the gap between your finger and the screen, allowing the touch to be registered. From my understanding the tip has to be more bulbous to ensure enough contact is made to register the connection.