A scaled-down cake, such as a cupcake, will have more surface area per kg of cake batter. There is more surface area exposed to the heat, or to put it another way, more of the batter is closer to the surface. Cooking will proceed more quickly, and the cupcakes will be burnt if they are cooked for the time specified for an equal-mass cake.
Work Step by Step
This is discussed on pages 236-237.