Why do I not gain much weight?

I'm 21 years old, in general I have a healthy, balanced diet, I go to the gym twice a week. However, I do have one or two sweet things almost every day. I am happy with the way I look (I'm 5ft 11, and I weigh 75 kgs). I don't want to put on any more weight, but I'm just wondering, why am I not putting any on. I'm not super skinny, just average, but not plump!