Are you sure about that?
We operate both B744 and B777 aircraft on domestic flights, the extra seating on the 744 doesn't theoretically pay for the additional fuel.......
Even if the fuel econ some of these newer twins are performing with is narrowing the gap, there are still additional aircraft operating costs, maintenance costs, landing fees etc.
Granted I proposed a theoretical situation myself, the larger the aircraft the larger the savings. Another thing is routes and slots, now we are talking about exploiting a market to it's fullest with an A380.
I was around on the other hand to watch every 747 acmi freight operator out there get hit hard when trade started to slow down circa 2000/2001. Watched ATLAS allmost go under as their customers started looking for DC-10 and MD-11 operators because they could come closer to filling a smaller jet. ATLAS had somewhere around 30 aircraft at the time.