You are young. Your growth cartilages is probably not yet complete, so your weight/height will fluctuate, I would not deal so much. It also depends on the role you are playing. If it is on the defensive post (sorry, I do not know exact terminology), there is a higher body weight assumed, no matter in quality.
Rationally written strength training is the most appropriate approach for rugby players. For example, my modified nonlinear periodization is effective. You will gain muscles, strength. Of course, if you eat properly, you lose body fat.