There is no code for the Barbarain AI grouping that looks at the size of your forces. Rather each barbarian unit is given an 'inherent' ability to group other barbs under them, up to 5 units (which should be quite rare). There
shouldn't be any way for more barbarian units to group together than that, but I'll be keeping an eye out. I've looked at the code again and don't see any bad math.
I can think of a possible loophole where one group is following another group to merge with who in turn is also moving to merge to another group. But even if that is even a possibility to happen (and I'm not sure it is), it would be god-awfully rare by my guess. If I see it actually happening, I've got some options to stop it.
I agree about the lizardmen, I'll take care of that. Might allow it somewhat later in the game though (gotta keep it interesting

)
And the new code will keep groups smaller in the early game until there are a significant number of cities built.
Oh, and if anyone sees barbs doing something like grouping too big, or never wanting to attack a city, etc., post a save game for me to look at. That'll be the best way to possibly track down any odd behavior, as it's often difficult to replicate the same issue.