Hi,
I wouldn't call this problem solved. I have to keep resetting the params field about every 2 weeks on one of my sites so that user accounts don't get a params field so large that you can't open up their CB or Joomla user account. I get a fatal error due to the user record being so large it can't be accessed in MySQL.
This seems to be happening to user's that update their CB Profile quite often. It seems that updating your CB Profile is somehow effecting the params field of the Joomla user record params field, making it grow large. The data in the params field still seems to be valid JSON format, just huge.
Sure would be nice if someone could find out what is causing this issue.