I can’t give you a statistically based answer because I’m not a mathematician, but what you have noticed is correct. If you double the number of lines, then you MORE than double the amount of traffic it can handle. That’s why it’s best not to split groups of lines into multiple trunk groups (eg. one for incoming and one for outgoing) at least from a traffic engineering point of view anyway.
Perhaps someone else can offer a more scientific reason?
James.