A big route & fleet expansion came with the aquisition of Morris and the integration of them during and after 1992, but unfortunately the Annual Reports only go back to 1994. Details for expansion for that year are here http://www.swacargo.com/investor_relations/swaar94.pdf
and lists 199 aircraft. From '91 (when Midawy Airlines went under) to the end of '94 I counted delivery of 43 a/c, so I figure SWA fleet size would be about 156 in 1991.
It's almost as if the company doubles in size every five years. In fact "Shareholders saw their Southwest holdings increase 10-fold from 1980 to 1985. After 1990, Southwest doubled again." (from Beating the Street by Peter Lynch, Simon & Schuster 1994)
You can see details of SWAs expansion by year under that very word in the index of in their annual reports that can be found here- http://www.swacargo.com/investor_relations/annual_reports.html