Wikipedia on "Cupcake"

Wikipedia entry on "Cupcake"

Cupcakes evolved from muffins during the nineteenth century, and gained the name "cup" cakes or "measure" cakes because of the use of the practice of measuring the ingredients using a standard-sized cup instead of the previous practice of weighing the ingredients. The fact that these little cakes were baked in cup-shaped muffin baking containers gave double-meaning to the term "cup cake", and was probably the main factor for "cupcake" becoming the U.S. standard term.

It is also possible that cupcakes came into being simply as smaller versions of the Victoria sponge cake, as the mixture required is exactly the same. The mixture is also the same as the quarter cake recipe, so called because it is made up of four ingredients in equal ratios; butter, self-raising flour, eggs and caster sugar.