Why am i still skinny and not fat?

I am 15 years old and 4'10 and 90 pounds. I am almost done growing. I eat soooo much more than someone my weight should. My average day consists of a bagel for breakfast, 2 pieces of pizza, a chocolate chip cookie, and a pack of cheez-its for lunch, chips, doughnuts, goldfish, and mountain dew when i come home, macaroni and cheese and chicken nuggets for dinner, and ice cream and pizza rolls before bed. That is just average for everyone in my family and we are all very tiny. My friends family eats way less and way healthier than me and they are all chubbier. I was just wondering what causes this?