Because if the capacity of a military canteen is too small, it will hold less water, which is not cost-effective. If the capacity is too large, it will be heavy and bulky, which will affect carrying. The material used in military canteens is relatively sturdy and can be used to hold green tea or other beverages.
The capacity accumulation of military canteens is also summarized and designed based on many years of experience. Usually, one liter of water can meet a person's daily drinking water needs (excluding water obtained from meals), and the army can also replenish water during marches, so the 1 liter capacity is the best balance between capacity and weight.