I've thought this too. You need to balance the size of your Amp.hr consumption against the lead acid battery bank size. Too small a bank and you put more deep cycle discharges on it and reduce their life. Too large a bank and it becomes more onerous to recharge them full every few days and again, their life is reduced, with an increased cost, since you now have to replace more batteries at once. There is a sweet spot between the two, but I don't think there is a simple equation to find it. More is definitely not always better and for some boats, reducing the bank total capacity might help with lifespan and total cost of ownership.