Thats due to the attachment of the long head versus the medial and lateral heads.
The med/lat heads attach to the humerus (upper arm bone) and insert onto the ulna of the forearm. The long head, on the other hand, attaches to the shoudler blade (right below the head of the humerus). The long head also assists in shoulder extension (ie: chin ups, neutral grip pull ups).
Now, if want to target the long head..which I recommend because it is the only of the three that is not adequately worked during pressing movements...you need to flex the shoulder (bring your arm up straight infront of you until it is parallel to the floor while standing) in order to increase the leng/tension relationship and fully recruit the long head.
Therefore, lying triceps extensions (keeping your shoulder blades retracted and not allowing your elbows to flair out) will fully recruit and work the long head of the triceps. Which, consequently, is also the largest and makes up a great of the girth of the upper arm.
Br