Personally, I could care less about total size of any single airline.

If UA wants to market on size, it should market the strength/coverage of the Star Alliance. To me, it matters less how far UA metal can take me and more how far *A can.

If WN wants to market on size, their focus should be on the fact that they are large and concentrated entirely into one tiny footprint. That means if you want to fly within this one country, chances are better that WN frequent nonstops to support you. That's the big value prop of Southwest: door-to-door on short flights, they nearly always have more options to choose from and they get you there faster.