It's just Moore's law in action. Basically what would have been a top notch ~100W+ server chip 10 years ago is now going into a 5-10W phone/tablet chip e.g. The Alpha EV6/7 -> Apple A7. This is why the Riscs led by ARM are having a mini-resurgence but I suspect lower process sizes will work against them again as x86 has more issue width headroom to play with going forward.