Madmacs,
Criteria is market capitalisation and liquidity. BTA easy makes it into the S&P ASX 300 on both counts.
See the following pdf entitled S&P ASX Index Methodology...
http://www2.standardandpoors.com/servlet/Satellite?pagename=sp/Page/IndicesBrowseMethodologyPg&r=7&l=EN&b=4&s=6&ig=42&i=78&si=&d=&xcd=ASX300&f=3