There is nothing wrong with eating small portions, as long as you do it multiple times per day. You might try substituting healthier options for your snacks, like nuts instead of chips or fruit instead of cookies, or drinking meal replacement drinks like Ensure. Your family should mind their own business, but it they don't, tell them that eating big meals makes your stomach hurt and that you are being careful to get enough calories and nutrition from multiple small meals so they don't need to worry about you. 