Don't lose sight of what "Market Cap" means -- It is "Market Capitalization," the current market value of its stock.
With all the reported losses at the other carriers, and the resulting loss of confidence in their stocks, it's understandable that their stocks have gone into the toilet. OTOH, SWA has reported actual profits, so confidence in their stock remains high.
Market Cap fluctuates daily. Some days, Microsoft has been "worth more" than GE (in recent times, the "largest" US company in terms of Market Capitalization). The physical assets and number of employees at MS are an order of magnitude less than that of GE, however...