You are eating less than what you ate to gain weight so you are therefore either out of a surplus or at a smaller surplus. Metabolic rate varies between individuals so what is "low" for one may be "high" for another. Half gallon of nonfat or lowfat milk is a pretty sustainable amount long-term. The caloric load isn't very high (~600 calories with 64 g protein and 86 g carbohydrate). I'd even contest that a gallon may be fine for some if done with lowfat or nonfat. In your case, are you still making progress under the bar?