The size of the city isn't what counts, it's the size of the MSA that counts.
1912 - Fenway Park - Boston Red Sox --> Boston-Cambridge-Quincy, MA-NH MSA - 4,522,858
1914 - Wrigley Field - Chicago Cubs --> Chicago-Naperville-Joliet, IL-IN-WI MSA - 9,569,624
1962 - Dodger Stadium - Los Angeles Dodgers --> Los Angeles-Long Beach-Santa Ana, CA MSA - 12,872,808
1966 - Angels Stadium - Los Angeles Angels of Anaheim --> Los Angeles-Long Beach-Santa Ana, CA MSA - 12,872,808
1966 - Oakland-Alameda County Coliseum - Oakland Athletics --> San Francisco-Oakland-Fremont, CA MSA - 4,274,531
1973 - Kauffman Stadium - Kansas City Royals --> Kansas City, MO-KS MSA - 2,002,047
1989 - Rogers Centre - Toronto Blue Jays --> Greater Toronto Area GTA - 5,555,912
1990 - Tropicana Field - Tampa Bay Rays --> Tampa-St. Petersburg-Clearwater, FL MSA - 2,733,761
1991 - Chicago White Sox - U.S. Cellular Field --> Chicago-Naperville-Joliet, IL-IN-WI MSA - 9,569,624
1992 - Baltimore Orioles - Oriole Park at Camden Yards --> Baltimore-Towson, MD MSA - 2,667,117
1994 - Progressive Field - Cleveland Indians --> Cleveland-Elyria-Mentor, OH MSA - 2,088,291
1994 - Rangers Ballpark in Arlington - Texas Rangers --> Dallas-Fort Worth-Arlington, TX MSA - 6,300,006
1995 - Coors Field - Colorado Rockies --> Denver-Aurora, CO MSA - 2,506,626
1996 - Turner Field - Atlanta Braves --> Atlanta-Sandy Springs-Marietta, GA MSA - 5,376,285
1998 - Chase Field - Arizona Diamondbacks --> Phoenix-Mesa-Scottsdale, AZ MSA - 4,281,899
1999 - Safeco Field - Seattle Mariners --> Seattle-Tacoma-Bellevue, WA MSA - 3,344,813
2000 - Comerica Park - Detroit Tigers --> Detroit-Warren-Livonia, MI MSA - 4,425,110
2000 - Minute Maid Park - Houston Astros --> Houston-Sugar Land-Baytown, TX MSA - 5,728,143
2000 - AT&T Park - San Francisco Giants --> San Francisco-Oakland-Fremont, CA MSA - 4,274,531
2001 - Miller Park - Milwaukee Brewers --> Milwaukee-Waukesha-West Allis, WI MSA - 1,549,308
2001 - PNC Park - Pittsburgh Pirates --> Pittsburgh, PA MSA - 2,351,192
2003 - Great American Ball Park - Cincinnati Reds --> Cincinnati-Middletown, OH-KY-IN MSA - 2,155,137
2004 - Citizens Bank Park - Philadelphia Phillies --> Philadelphia-Camden-Wilmington, PA-NJ-DE-MD MSA - 5,838,471
2004 - PETCO Park - San Diego Padres --> San Diego-Carlsbad-San Marcos, CA MSA - 3,001,072
2006 - Busch Stadium - St. Louis Cardinals --> St. Louis, MO-IL MSA - 2,816,710
2008 - Nationals Park - Washington Nationals --> Washington-Arlington-Alexandria, DC-VA-MD-WV MSA - 5,358,130
2009 - Citi Field - New York Mets --> New York-Northern New Jersey-Long Island, NY-NJ-PA MSA - 19,006,798
2009 - Yankee Stadium - New York Yankees --> New York-Northern New Jersey-Long Island, NY-NJ-PA MSA - 19,006,798
Under Construction
2010 - Target Field - Minnesota Twins --> Minneapolis-St. Paul-Bloomington, MN-WI MSA - 3,229,878
2012 - Marlins Ballpark - Florida Marlins --> Miami-Fort Lauderdale-Pompano Beach, FL MSA - 5,414,772
MSA's skipped by baseball???
Riverside-San Bernardino-Ontario, CA MSA - 4,115,871
Portland-Vancouver-Beaverton, OR-WA MSA - 2,207,462
Sacramento-Arden-Arcade-Roseville, CA MSA - 2,109,832
Orlando-Kissimmee, FL MSA - 2,054,574
San Antonio, TX MSA - 2,031,445
Las Vegas-Paradise, NV MSA - 1,865,746
San Jose-Sunnyvale-Santa Clara, CA MSA - 1,819,198
Columbus, OH MSA - 1,773,120
Indianapolis-Carmel, IN MSA - 1,715,45
Charlotte-Gastonia-Concord, NC-SC MSA - 1,701,799
Virginia Beach-Norfolk-Newport News, VA-NC MSA - 1,658,292
Austin-Round Rock, TX MSA - 1,652,602
Providence-New Bedford-Fall River, RI-MA MSA - 1,596,611
Nashville-Davidson–Murfreesboro–Franklin, TN MSA - 1,550,733
The only teams with less population in their metropolitian statistical area than San Antonio are Kansas City (very, very close) and Milwaukee (which attracts fans from throughout the state). San Antonio is just barely below Cleveland and Cincinnati in population. But they have a history of major league baseball that extends over 100 years. There are four other metro areas larger than San Antonio. San Antonio may attract fans from the Austin area, but only on weekends.