We've come up with a recipe for a cake that has four different layers (a dough base plus fillings) and is then covered in chocolate mousse. We tested that recipe as a small sample and concluded that the ideal proportion was to have 70 grams of base layers for 80 grams of chocolate mousse, creating a tight cylindric 150 gram cake. So far, so good.
We then proceeded to make a bigger version, with a wider mold, which would weight 1.88 kilograms, so we scaled our recipe and found out that 875 grams of base layers for 1kg of chocolate mousse. But the problem is, the mousse layer becomes so large that it collapses.
Fixing this is fairly obvious: use less mousse. Maybe, since the mousse is less dense than all the other layers, increasing its total weight has a much bigger impact in terms of volume. But how can we define our recipe in terms of volume rather than weight, then?
I feel like we're missing something very obvious here, but if the proportions of the recipe are the same as the small, initial version, why do we get such different results? How can we scale the recipe in order to achieve a consistent amount of mousse, regardless of the size we attempt to make (i.e. what kind of math is required to make a 1kg, 2kg and 3kg cake from one single recipe / set of proportions, considering the form factor stays the same -- still round, same height, just different diameter -- if it's even possible to do so)?