NEW YORK (TheStreet) -- A strong proponent of capacity discipline for the big three carriers, Wall Street is giving a pass to smaller carriers led by JetBlue (JBLU) - Get Report and Virgin America (VA) .

Both carriers benefited from positive passenger revenue per available seat mile (PRASM) growth during the second quarter, a time when the big three carriers all reported diminished PRASM. The importance of PRASM is currently a topic of intense debate in the airline industry.

Virgin America shares rose 7.5% on Thursday despite forecasting fourth-quarter capacity growth of 9% to 10%. For the second quarter, the carrier reported earnings growth of 134%, beat estimates and produced passenger revenue per available seat mile growth of 0.6%.

"People were expecting weaker results given the highly competitive markets we are in," said Virgin America CEO David Cush in an interview. "We're holding our own (in PRASM) but also we beat estimates (and) our income was up."

On the American (AAL) - Get Report earnings call a week ago, CEO Doug Parker sought repeatedly to make the point that PRASM is not a tell-all indicator -- if costs are down, then revenue can decline with no negative impact on profit. In the second quarter, American's fuel cost fell by $1.24 billion from the same quarter a year earlier. "Change in RASM does not equal change in value," Parker declared.

Cush agreed that PRASM "should not be the sole thing we are measured on." As for capacity growth, he said that with the economy growing by about 2%, selected "increases in capacity make a lot of sense in this environment."

Wolfe Research analyst Hunter Keay wrote Thursday that Virgin's PRASM continues to outperform the industry despite competitive headwinds, "though those headwinds abate in 4Q."

Virgin "also beat the high end of its initial PRASM guide while everyone else missed, but that was likely do to a conservative guide" as well as a one-time, frequent flier-related revenue benefit. Cost guidance also improved, he said.

JetBlue shares gained 4.5% after the carrier reported earnings on Tuesday. JetBlue matched estimates and reported PRASM growth of 1.4%.

Stifel analyst Joseph DiNardi wrote that "JetBlue's stock outperformance continues to be driven by its above-industry average PRASM performance.

"While this is, to some degree, a function of more favorable competitive capacity trends (which we do not see as entirely sustainable), it's also due to the maturation of its network and a lack of exposure to competitive capacity 'hotspots.'," DiNardi wrote.

Each of the big three earnings calls included mentions of airports where competitive pressures have resulted in diminishing yields. First, Delta (DAL) - Get Report President Ed Bastian cited Chicago, Dallas and Orlando. Jim Compton, United (UAL) - Get Report chief revenue officer, cited Dallas, Chicago and Houston. American (AAL) - Get Report President Scott Kirby cited Dallas and Miami, but surprisingly, Kirby said, "Chicago yields actually did better in the quarter than our domestic average."

 Asked about Orlando, where JetBlue has a major presence, Executive Vice President Marty St. George on the earnings call responded, "We've certainly heard the commentary about airlines but we're very happy with what we're seeing in Orlando."

Virgin America said PRASM fell in Dallas, but outside of Dallas and New York's John F. Kennedy International Airport, PRASM gained 5%.

"We're taking a long-term view," Cush said. The growth in the market, led by Southwest expansion at Love Field, "was a tremendously disruptive event, and the market is behaving as we would expect after a 30% increase in capacity. It will improve considerably."

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.