I agree with Mackin, this is fun...
If anyone cares about the original question, the different capacity batteries don't make a bit of difference. That is until one of them drops a cell and loses voltage. What happens then is very high amp transfers between the two mismatched batteries resulting in the low volt one boiling off and possibly blowing up....
I agree - kinda refreshing to get back to the QUESTION (-:
You can model it EXACTLY as several batteries and the internal resistance of each.
Both the charge and discharge rate of each is some inverse function of its internal resistance - which, BTW changes with its STATE (level) of charge.
They can all have different internal resistances, in fact larger "capacity" batteries will typically have a lower internal resistance than smaller ones - since internally they are just built from more sets of "plates" in parallel.
If any one goes to a very high resistance "open circuit" it neither draws current nor supplies current.
It ceases to exist as far as the others are concerned.
Since the VOLTAGE is regulated this doesn't lead to overcharging of the other(s).
If any one develops a very low internal resistance - an internal "short circuit", it becomes a load (current sink) to the others.
It won't necessarily destroy them, though it is likely to run them flat very quickly if the bank is not being charged frequently.
If its internal resistance is merely "low" (not VERY low) it may not show when all are on charge - i.e. if the alternator can keep up with the added current draw.
So, same old same old.
Bad batteries are BAD to have around period - whether individually or in parallel.
Just get 'em outta there.
No "what if" or "how come" about it, they're BAD NEWS.
They HAVE TO GO.
