in bodybuilding it's called "inner bicep thickness", you need to shift focus from the long head (which builds the outer peak you see from the side) to the short head of the biceps, plus the underlying brachialis muscle

Preacher Curls (EZ Bar or Dumbbell)

Concentration Curls

Reverse Grip EZ Bar Curls

Hammer Curls (for Brachialis Width)

Above listed exercises help target this.