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Milk is a liquid of heterogeneous in character containing fats and water at normal temperature. When milk is heated fats containing in the milk get boiled along with water converting fats into cream and floats over the boiled milk in the form of many small bubbles forming a thin creamy layer over the milk surface in the container. 

During boiling water contents in the milk produce steam and trapped under the thin film on the top surface of the boiling milk. Continuous heating and boiling of milk in the container will create high pressure below the thin membrane formed by the cream film resulting overflow of milk.

Since the pure water is a homogeneous substance the steam produced during boiling evaporates stem in the air without any obstruction as seen in case of boiling milk. That is why boiling pure water continuously will not overflow the container.