Southwest is estimating that its passenger unit revenue increased 14% to 15% year-over-year in January.

The gain is eye-catching in comparison to other U.S. carriers: Continental this week reported a 1% to 2% decline in passenger unit revenue for January and JetBlue reported a 2% drop.

Southwest is getting a big part of its unit revenue boost from a surge in traffic despite cuts in capacity. For January, traffic increased 7.1% following a 6.7% cut in capacity, resulting in a 9.3-point increase in load factor to 72.1%.

But Southwest executives insist the airline’s lack of a fee on the first or second checked bag – plus its marketing campaign promoting the difference from competitors – also contributed to unit revenue gains.