Sessions Table Getting Huge
While waiting for an answer, I found the solution at this blog.

All you need to do is to edit the settings.php under /sites/default folder.

Here are the two variables that need to be set:

ini_set('session.gc_probability', 1); 
ini_set('session.gc_divisor', 100);


Be sure to change also the cache expiry of the following variable according to your needs:

ini_set('session.cache_expire',     3600);
ini_set('session.cookie_lifetime',  3600);
ini_set('session.gc_maxlifetime',   3600);


The default is 200000. 3600 is equavalent to 1 hour.