The construction of the processor chips has changed. The size of the printed semiconductors is thinner now, and the resolution of the printed semiconductors themselves has improved. This is called the "process" size, that's the pixel size used to draw the individual parts on the chip die. Current commercial CPUs are 14nm process, but Intel is heading toward 5nm by 2020. 10 years ago it was 10 times bigger. The smaller the process, the less current it consumes. So the voltage of the chips can be lower and the heat they waste is lower making the whole product smaller, cooler, lighter, etc. Cell phones are driving this, of course. It affects memory chips, GPUs, everything getting smaller, so they can fit it all inside your eyeball.
http://www.10stripe.com/articles/what-does-process-size-mean.php
