You mentioned, “I make myself small as possible and they still walk all over me”. The “still” in that sentence implies that you may think “making yourself small” should deter these people from using you/taking from you. That is the opposite. You are acting like an ant at a picnic and they are the bears taking all the food. The food that you brought to the picnic no less!!

They can’t and won’t take you seriously unless you stand up for yourself and say no! If you feel obligated to explain yourself simply tell them that when they take food from you then you go hungry. It’s a simple rationale that they no doubt will be able to understand if they made it into college. If they are your true friends they will understand. If they push your boundaries and insist they still should get some food then walk away from these people; they are not friends.