Milk is the heavier liquid when it is compared to pure water. Some milks actually contain 87% water and other subsistence that weigh a lot more than water. Therefore, milk weighs more than pure water per gallon. Whole cow's milk is also more dense than pure water as it contains dissolved protein and sugar. Whole cow's milk is roughly only 2 ounces heavier than a gallon of pure water due to the protein, fat and sugar.