I think it is relevant to the op so I would add this -
think of what happens to resting voltage when you pull 50% out of a battery vs 25%. The only reason both (in his bank) remain at the same voltage is because they are interconnected - the small one is actually competing with the larger.
No matter how you slice it, when you pull amps out of a bank, each battery loses half the amount. When one is smaller than the other, it suffers disproportionately.