Couldn't agree more. I often make double or triple portions in one go, and have found that a gradual diminishing of quantities when upscaling is the way forward.
For a double Madras, for instance, I'll use 1.5tbsp of chilli powder instead of 1tbsp, and for a triple I'll use just under 2tbsp instead of 3. (Same sort of ratio applies to other ingredients, too.)
I will admit to finding it a little baffling. After all, if you cook two or three Madras curries separately then mix them together in one bowl at the end, all is fine.
I'm not sure how the 'science' of it works, but it just works!