Hello their, Yesterday I created a forum [phpbb] and created 4 forums and had 2 users. and my MySQL database size increased to 2.1 MB ! Is it OK ? I am running a wordpress blog with around 100 posts, 200+ tags and much more content but it took only 2MB size of MySQL.
I don't know much about phpbb, it might save much information/settings in DB. It is not a big size that you should worry about. Keep an eye on it and see the growth.
in that case ,phpbb must automatically generated many revevant tables , i dont have any ideas about it jet coz phpbb is new to me . if things goes worse . i suggested you to consider druapl ,joomla which is integrated with forum too.