In order to gain an inch on your hips, you will need to eat a little more.  You need extra calories to help give your body the fuel it needs to increase the size of your hips.  Just like with weight loss, you cannot spot treat with weight gain.  You will need to gain weight safely and slowly over your whole body to see your hips increase in size.  You can do this by adding about 250-500 calories to your typical daily intake. For example, if you currently eat 1,800 calories per day, try increasing your daily caloric intake to 2,050 - 2,300. Try using a food journal or journaling app on your smart phone to calculate your current calorie intake.  That way, you'll know what level to aim to gain weight. In order to increase your total calorie intake, you'll need to increase how much you're eating.  You can increase your portion size or eat more frequently throughout the day.  One of the easiest and more simple ways to gain weight, is by eating more frequently throughout the day.  Try adding in a 4th small meal or adding 1-2 snacks a day in addition to your typical 3 meals per day. Eating more frequently can help prevent you from getting overly full from larger meals and keep you feeling energetic throughout the day. Another factor you'll need to focus on is the types of foods you eat.  You need to add in foods that are higher in calorie so that those extra meals or snacks add up to be the needed 250-500 calories per day.  Calorie-dense foods will help you meet a higher calorie target a little more easily.  For example, adding in a small salad as a fourth meal will only add about 100 calories or less. Instead, focus on foods like that are higher in calories.  Foods that are higher in protein and healthy fats are a good place to start.  Try:  nuts, avocados, dairy products, eggs and fatty fish. For example, you can add healthy calories with snacks like:  peanut butter and an apple, 2 hard boiled eggs, trail mix, or full fat greek yogurt with nuts. Avoid adding calories with unhealthy foods like sweets, fried foods,...

TL;DR in Vietnamese:
Tăng lượng calo nạp vào hàng ngày. Ăn ba bữa mỗi ngày và một hoặc hai bữa ăn nhẹ. Chọn các thực phẩm bổ dưỡng và giàu calo. Tập trung vào protein.