In 2015, Delta had a total of 209.6 billion revenue passenger miles, surpassing United's 208.6 billion. American (including the now-defunct US Airways) hasn't released its numbers for 2015 yet, but with 204.7 billion revenue passenger miles as of the end of November, it's likely that AA won't be overthrown. Delta's continued expansion both at home and abroad, as well as the continuing struggle of the 2010 United-Continental merger are possible explanations for Delta's edge over United.
While travelers interested in the points and miles game might not be too keen on Delta and its constant changes and devaluations, it's hard to argue that the carrier doesn't have a good network of appealing flights for travelers. As United bolsters its customer service and seeks to grow its own route map (especially with new long-haul Dreamliner service), it's possible that the airline could reclaim the spot in 2016.
H/T: Bloomberg
