How to Gain Bicep M?

So I am sort of confused at this point. I have been working out for the past six months and have made large gains strength wise. For bicep curls, I am able to curl 35 lbs to 40 lbs each arm for around 8 to 6 reps respectively (3 sets). In my opinion and compared to my peers, the weight I curl is pretty good. However, my gain in m is very marginal and my arms are still pretty small compared to others. I weigh around 150-155 lbs and am 17 years old. So my question is, how come I have made such gains in strength and can lift more than others, but my arms are still relatively small?