I first started flying Delta for work in about 1996, I remember around that time they were the first airline to ever carry over 100 million passengers in a year (and by extension, they carried the most passengers of any airline). At some point after that and before industry consolidation kicked into high-gear Southwest passed them in passengers carried (but not passenger-miles, another measure that can be used), and as the data you pointed out shows, Southwest has now dropped back a few spots behind the consolidated legacy carriers.