I suppose, to go back to your first question, think of it like this.
If you have a 14 seat plane, and you get the price just at 100%, what you are doing is finding the top 14 richest, most desperate people at ATL who absolutely have to get to ORD. They will therefore pay a very high price. If you increase the capacity to 28, you have to find the top 28 people willing to pay a small fortune to go between these cities. However, to get the 100% loadfactor, to get the 28th person to fly with you, you will have to charge a lot less than to get only 14 people.
You will also notice that, as the price drops, it large increases in the number of seats have smaller impacts on the price (e.g. going from 10 seats to 20 seats might get a drop in fares from $1000 to $700, but going from 170 seats to 180 seats might only drop the price $150 to $140). This is because the majority of people are not willing to pay such high fares (as $1000), so there is a very limited number of customers, and an increase of 10 seats means prices will have to drop considerably to keep up loadfactors. However, a much larger number of people are willing to pay $150 for a seat, so even if you increase the aircraft size 10 seats, a small price drop, with such a large number of potential customers, can fill these seats.
I hope this explains everything.