Sorry, I wasn't clear enough - by surface area I was talking about the surface of the limestone exposed to the water, not the tank itself.
For Rift Lake and brackish fish I tend to use aragonite substrates with over-engineered RFUG filters. Even though the material may get coated with biofilms, the substrate does help buffer the water up. Using calcareous rocks you have much less rock surface exposed, but it still contributes. Either process will provide some additional minerals to the water.
The bioload effect occurs because normal nitrification destroys a bit of the carbonate/bicarbonate buffering in the water. This is compensated by water changes replenishing the carb/bicarb buffering, but also by some minerals dissolving from rock and substrate if they are calcareous. The substrate or rock contribution is not awfully fast, so if you have to do large volume frequent water partials due to high bioload, you may not see as much buffering being added.
If you have naturally low KH and pH water, the changes from the substrate and or rocks will be more noticeable - because it does not take much change to see a difference in "soft" (low alkalinity) water. With harder source water, the change will not appear to be as great. This just means that if your source water is 2 KH and you add sufficient carb/bicarb to double it to KH 4, this is easily detected. If your water is KH 8 and you add the same amount of additional carb/bicarb by substrate or rocks to KH 10, it is not as big an apparent change (1/5 more versus twice as much). Also, in practice the lower pH water will dissolve more mineral in the same time, as the solution rate is pH dependent.
The bottom line is that calcareous material will affect your water, but the end result in your tanks cannot be predicted, as it is dependent on too many factors.
HTH