The rule I learned from my Mom was to stll add some more of those ingredients when doubling, but to use less than a full double amount. IIRC, the ratio went something like: Double - 1.5x the normal amount of Baking Powder/Soda Triple - 2x the normal amount Quadruple - 3x the normal amount More than quadruple - not a good idea, make multiple batches instead. As I mentioned, I learned this from my mom, who learned it from her mom, who learned it from her mom, who learned it from a dinosaur. It is therefore entirely anecdotal and almost folkloric information. Any basis in actual science has been lost in the mists of time. It does work, though.