While conceptually you could "just make the tank bigger" to...

While conceptually you could "just make the tank bigger" to increase capacity, there is likely to be more to do than that. Since you are depositing Zinc onto the anode (electroplating) during discharge it is probably sized for the amount of zinc in the solution. Adding extra solution without changing the anode size or configuration could lead to excessive deposit thickness and dendrite formation that could possibly internally short the battery.
If anyone knows more about the battery chemistry and working perhaps they can confirm or refute this.