Why do I gain weight when I eat less protein?

Lean proteins are a great way to fill up on less calories, which leads to decreased portion sizes and thus weight loss. If you are eating less protein, you tend to eat more carbs. Carbs, especially those that are low in fiber, don't fill you up as much and you tend to want to eat more. Naturally, carbs hold onto some water which might also cause initial weight gain if transitioning from a high protein diet. Speak with your dietitian about appropriate amounts of protein and carbs for you specifically.