Ian and merrybaker's arguments and references are so full of holes I barely know where to begin so I won't bother. Suffice to say that my argument that you scale proportionally is correct. If you want ten times the amount of curry that you usually cook, just multiply everything by ten.
What I should have stated, although I thought it was blindingly obvious and so didn't, is that if you scale up and use the same cooking methods that you would use for the smaller portion, then yes it will probably turn out different.
For example, say your recipe calls for lightly browning one onion. Great, you think, you get your big pot out and throw ten onions in because you've decided to scale up by ten. At the end of the cooking time you find that the onions don't quite have the same caramelisation that you find with the small batch. And of course they won't, because you're not cooking like for like. What you've done is sweat the onions because you've got too many in a bigger pan and not enough heat. You've changed your technique!
So you see it's not that the scaling up by ten that hasn't worked, it's that you've cooked it differently. Therefore let me restate my initial claim:
To the question "when scaling up a recipe, do I scale up the ingredients proportionally", the answer is a very incontrovertible YES, providing you have the wit and common sense to realise that you will need to modify your cooking method to suit.
Also cooking is definitely a science, it's just that those who have mastered it make it look like an art.
