How can my thin friends eat and not gain weight but when I eat, I gain weight? :(

We all eat healthy but even when my friends east junk food, they stay thin but for me I gain weight so fast. I gotten fatter in middle school and my friends got skinny. I lost weight in high achool but gained it back. My friends are slim now. I have to watch what I eat. My friends do too but they stay so slim. No fair :(
I'm working to lose weight. I'm naturally chubby/curvy. But I'm 6ft and 220lbs.