Yes, they normally get much bigger than that. Thats about average for a southern coon, but many northern coon top 30 lbs. Check out this thread for some pics of some really big coons.
http://www.trapperman.com/forum/ubbthreads.php/topics/1651142/Re_Coons_coons_and_only_coons.html
http://www.trapperman.com/forum/ubbthreads.php/topics/1651142/Re_Coons_coons_and_only_coons.html