If your diet was on point, you would be gaining weight, good or bad but you are not eating enough. Its simple math really. I would have to see your diet, pics and understand what your lifestyle is like, but this is not adding up. You are either burning a lot of cals due to a lot of output, like a physcial job or you just have a fast metabolism. Regardless, if you are eating above maintance, you will get bigger, either muscle or fat. Thats just the way it is. Sorry I cant say more without seeing more details, but my gut tells me that you are not eating enough and there is a chance you might not be giving your body enough reason to grow with your training. Diet and training is where I would start. You will grow with or without steroids if these things are in order