Gaining weight on 1200-1300-1400 calories a day

I am tall too.
I am 5'9" and in my 120's.
Whenever I eat 1400 or 1300, I gain fat weight around my stomach and thighs.
I am supposed to eat 1700 to maintain but I will gain so much on that amount. I don't know what is happening
My thyroid is fine, I am young so my metabolism should be too. I'm 18