The problem is inefficient curling technique. It's not a lack of compound movements or needing to add more overall bodyweight.
1. Start the curl with the correct body position. A straight body with the shoulders back and down increases loading on the biceps.
2. Tense the triceps, abs, and glutes before starting a curl.
3. Contract the biceps as hard as possible while lifting. When the arm is fully flexed, finish the curl by rotating the shoulders up, bringing the elbow higher.
4. Do not use your front delts and traps to lower the weight. Make the biceps do the work.
The main reason why people are having trouble putting size on their biceps is simple: they don't really know how to curl. As a result, they're just not good at fully stimulating their biceps. The curling action is fairly simple, but some small details can make a huge difference in how effective the movement will be.
The position of your body when performing an exercise is important. Even during a "small" movement like curls, using a proper body position can totally change muscle recruitment. Without even knowing it, the first mistake almost everyone makes is tensing their traps. Their shoulders go slightly up and forward before they even start the curling motion.
The Optimal Starting Position for the Curl
1. The body is straight with the shoulders, hips, and knees in line.Do not bend forward or lean back.
2. The shoulders should be down and back, not forward and up.(This is also called a downward rotation of the scapula.) If you stay like this during the whole lift you'll increase the loading of the biceps.
3. The crook of the elbows should be pointing as straight forward as possible. Using a straight bar instead of an EZ-curl bar helps here.
Don't Lift Yet! Get Tense First.
It's also a mistake to do any exercise with a relaxed body. You need tension in the right places to optimize motor recruitment and performance. Even when you curl it's important to focus on tension in the right places. It's not just about flexing and extending your arms.
First, you want to tense/flex your triceps to get into the proper starting position. Flexing your triceps will allow you to fully straighten your arms in the starting position. This means that you'll have a slightly longer range of motion to load the biceps. It might only be an inch more, but over time this is enough of an increase in stimulation to make a difference. Remember, the muscle being stretched the most is the muscle being recruited the most. By using the triceps to stretch the biceps you'll increase biceps recruitment.
Second, tense your abs. Imagine you're about to get punched in the stomach. Tense your glutes as well and try to "grab" the floor with your toes. All that rigidity creates a super stable base from which to curl. The body thrives on stability. The more stable your body feels, the more it will allow you to recruit the prime movers you want to hit.
Now that you're all set to curl it's time to pre-load the biceps. Just prior to bending your arms, tense your biceps – the strongest isometric contraction you can. As soon as you reach that high level of tension you begin to curl, paying attention to maintaining the same posture and rigidity.