Making PhpDig store the content in SQL tables won't take up any less space (I guess unless you used compression). You can limit the storage of text content by changing this line in the config file:
Code:
define('CONTENT_TEXT',0);
This way no text files will be created. Otherwise I guess you'd have to make some fulltext tables, then go through the code, and everywhere you see an fopen (in phpdigWriteText (robot_functions.php) for example), change it to an SQL query.