The vacuum created by the radiator keeps the bottle from overflowing. The small amount in the tubing is not enough to cause an overflow when you open the rad cap. Remember, when the rad cools after shut down, it sucks fluid back through the hose into the rad as long as you don't have any bubbles in the hose.
I've had my chrome recovery bottle mounted to the crash bars. When installing, hook up the hoses and raise the bottle above the rad to remove all air from the line. I use clear tubing so that I can see. Once the line is bubble-free, I lower the can and attach it to the crash bar. If all the hoses are tight, it maintains a solid block of fluid.