"The shrink from 28nm to 20nm obviously reduces the transistor size, but with double the transistor count, the A8 could have come closer to the same size as the A7. Apple likely improved the packing of its 64-bit Cyclone CPU core, and some of the additional transistors could have come in the form of more cache memory, which is denser than logic used for CPUs and GPUs.
Apple devoted those extra billion transistors to improving CPU performance by up to 25% and speeding up GPU performance by up to 50%. At the same time, Apple is claiming power savings that allow it to sustain performance, while other processors start to thermally throttle performance."
http://www.eetimes.com/author.asp?section_id=36&doc_id=1323929