big.little is a disaster at 28nm because trying to fit four A15 cores in a phone just does not work. Basically what they have done to make this work is overclock the lower performance A7 quad-core up to 1.6 GHz and then under-clocked the higher performance quad-core A15 to 1.2 GHz to take over the work at the high end which is probably equivalent to say having 2.0-2.2 GHz on the quad-core A7. It would have been just more power efficient to have made this just a highly-clocked quad-core A7 in the first place and saved yourself all that leakage on the pointless A15s, Nvidia are doing this with the quad-core 2.3 GHz Tegra4i which uses A9s. This is what happens when you have amateurs (like Samsung) trying to do processor chips, you don't know what you are going to get until it comes out of the oven as they obviously have not thought this through properly beforehand. If you going to use A15 on a phone you should only 1-2 cores although you can do 4 on a tablet.