De forma simplificada (esto es, aproximada), en Cult of Mac:

All other things being equal — and as we’ll see later, all things are not equal – the main advantage ARM chips have over Intel ones is power management. But why are ARM chips so much more power-efficient than Intel’s that they can be used in the iPhone?

It’s because of a fundamental difference in the chips’ architectures. ARM’s RISC-based architecture has a distinct edge in power-efficiency over Intel’s x86, which was designed in the late 1970s. While computer architecture is a complicated thing, for the most part, RISC is more power efficient than x86 because it has to spend less energy figuring out where one instruction ends and the next begins.

With x86, an instruction to the chip can be any number of bytes. That means in any 64-byte chunk of memory, you can have any number of instructions… and as a result, a computer chip has to spend energy separating instructions before it can process them. With RISC, though, every instruction is 4 bytes: the chip knows that every 4 bytes, it can expect to see a new instruction. It doesn’t have to work as hard figuring out the grammar. Physically, this manifests itself in an ARM chip by allowing you to make your CPU cores smaller than their x86 counterparts, and for these CPU cores to draw less power.

Think of it like this. Which of the following sentences is easier for you to read?

Intelsx86architectureissuperraddude.

The cat sat and ate his hat.

Y como avisa al principio, ambos procesadores no se comportan igual ante el resto de criterios usados para compararlos. Los procesadores ARM son inevitablemente menos potentes y menos rápidos, y pierden en eficiencia frente a los Intel por temas de cache e input/output, lo cual equilibra bastante la balanza y les deja una ventaja mejor de lo esperado, en torno al 4% de ahorro energético. Más aún, la previsión es que Intel consiga igualar y superar la eficiencia de ARM antes del año 2015. En todo caso, el conceto es el conceto, y éste es el motivo por el que los procesadores móviles gastan menos energía que los habituales en computadoras «completas».