Dan,
They are limited by the number of gates they have at every airport they go to. MSP's LCC terminal, the Hubert Humphrey terminal, houses Airtran, Southwest, and Sun Country, and that limits growth potential. It's not really that big of a terminal from what I have seen, although I guess they could try to expand it. In ATL, Airtran has about one terminal's worth of gates (spread over two total terminals shared with ASA and some other airlines). Those gates are currently used to the max anyway, and if the SWA blowhards on here think they can increase turn times while at the same time increasing airplane sizes (717 and 737-700s up to larger 738s), then that will be interesting to watch. They don't have 737-800s now, but they state that they already know how they WILL do it, with more passengers to deplane and board, more FREE bags and cargo to off load, more time needed to clean the plane via the flight attendants, etc.
But, they say they can do it...... And Dan, watch out for them coming to Hawaii sometime within the next decade.
Bye Bye---General Lee