Is Weight Classified By Body Size? And How Do I Get Fat While Staying Healthy?

I'm 130 pounds and literally extremely skinny. I have been 130 since the 8th grade (last year) and haven't gained since then. But I did gain alot since 5th grade. When I started 5th grade, I was 60 pounds, but when I started 8th grade, I was 130. I have been skinny and stayed skinny, only growed... 8 inches?

My friend weighs around the same weight as I do, we're same the same height. The only difference is... HE'S HUGE!!!!! And sometimes as horseplay, he can easily push me down. But I can barely make him move. We get the same P.E. and exercise everyday. I even saw his weight on the weigh scale... I think it said 130?

My other friends are the same: 120's, 130's, and 140's. Most of them big, but they still over power me. Like according to one of them, I'm a "skinny bone jones." I'm basically as skinny as any nerd you'd see on tv. Besides asthma, I'm not sick or anything. And just outta curiosity... how exactly do I get fat or at least big while staying healthy? Wait to begin with... is 130 pounds supposed to be big? Because I don't even see where the f--- the fat is going in my body aside from the toilet.
Well sorry, not necessarily "fat" just big enough.