As per Charles law volume is directly related to temperature. So when the temperature of coolant rises, it expands and exerts pressure on radiator. At particular pressure (0.9 or 1.1 bar), the release valve of radiator opens and some coolant travels into the reservoir bottle. So if the reservoir bottle is already filled till the neck, it will throw away the extra coolant. It should not throw away the whole bottle. The pipe dipped into the reservoir bottle is connected to radiator and the pipe going out of reservoir bottle is connected to the lid of the bottle (its not going deep). So it is not supposed to empty the entire bottle. If its doing so, there must be some overheating or air leak problem.