I'm 16 years old, 5' 7", and about 165 pounds right now. My chest is about 41", my waist is about 28", while my arms are only about 13". Right now, I've been bulking up and my entire body seems to get bigger except my arms. I can deadlift 315, squat 290, and bench 220. I focus on compound movements for my arms like barbell curls, lying tricep extensions, and dips, and I keep my reps between 4 and 6. My arms just don't seem to grow at all.