The brachialis is the smaller muscle, which lies underneath the biceps.
There's also a little helper muscle tucked under the bicep called Brachialis.
This is one of the bicep exercises that pull double-duty in terms of targeting that helper muscle Brachialis mentioned earlier.
To make your biceps look big and proportionate, your bicep flex routine must include exercises for stimulating the two biceps heads, as well as the brachialis.