I'm just wondering how to keep muscles hard. I've noticed all of my friends who work out with me at the gym have harder biceps (just a muscle that everyone shows off) and we all do the same body parts on the same days. Meaning same bicep exercises, same amount of reps but they use 30-35 pound dumbbells and I use 25's. My friends are both about 155-160, age 16 and 17 and I'm about 131, 16 years old, I was wondering if it is the body fat difference or weight difference? If anyone has any answers please post them. Thank you.