If the phpFreeChat module has a lot of data in there, I'd look at the module to see what it's doing with $_SESSION. It may be storing data there it doesn't need to and/or not removing data later on when it doesn't need it anymore. It really depends on the modules you have installed. OG, for instance, stores the complete node of the current group a user is viewing in the session, which makes the session table get really big (far too big, IMO). Other modules don't store more than int or two in the session, if that.
OG used to do that, but has changed in D6 and about to be released D5. We only do that for authenticated users now, and it gets cleared out when user visits a non group page. Sheryl - just call session_save_session(FALSE) and Drupal will not save anything to sessions table. You should explore any ramifications of this for other modules.