I finally found where I read Southwest was the 5th largest carrier! It was in the press release when they announced Albany!

"Southwest Airlines, the nation’s fourth largest carrier in terms of Customer boardings, serves 55 cities in 29 states—this will increase to 56 cities in 29 states when Southwest begins service to Albany International Airport. The airline currently has more than 29,000 Employees and operates more than 2,500 flights a day. "

I wonder who's in front and who's behind.