Way too much has been made of being the largest airline. I always felt AA could have made it as a stand alone carrier now the chapter 11 has leveled the playing field. They have top tier hubs, a good reputation and many new planes on order. US on the other hand has spent the last few years looking for a partner, first DL,then UA, and it finally achieved its goal. While concentrating its efforts on merging, its domestic route map has not grown and neither has its widebody fleet grown very much.
WN and B6 have chipped away at US's stronghold, the Northeast and this merger was a do or die for Airways.
Size does not determine success. Good customer service and good employees do. B6 is doing very well in spite of its size, good management and a simple fleet make up are part of their success.
Now we all wait to see if Parker will squander this opportunity by injecting US customer service or building on the AA name and reputation, time will tell.